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import java.io.*;
import java.nio.charset.StandardCharsets;
import java.util.*;

public class kon {

	static String in1 = "4 5\n1 2\n2 3\n3 1\n3 4\n4 2\n";
	static String in2 = "3 2\n1 2\n2 3\n";

	static InputStream stringToStream(String string) {
		return new ByteArrayInputStream(string.getBytes(StandardCharsets.UTF_8));
	}

	public static void main(String[] args) {

//		InputReader in = new InputReader(stringToStream(in1));
//		InputReader in = new InputReader(stringToStream(in2));
		InputReader in = new InputReader(System.in);
		OutputWriter out = new OutputWriter(System.out);

		int n = in.readInt();
		int m = in.readInt();
		Map<Integer, Collection<Integer>> graph = new HashMap<>((int) 1.5 * n);
		Map<Integer, Integer> inDegrees = new HashMap<>((int) 1.5 * m);
//		Map<Integer, Collection<Integer>> ins = new HashMap<>((int) 1.5 * m);

		for (int i = 0; i < m; i++) {
			int from = in.readInt() - 1;
			int to = in.readInt() - 1;
			if (!graph.containsKey(from)) graph.put(from, new HashSet<>());
			if (!graph.containsKey(to)) graph.put(to, new HashSet<>());
			graph.get(from).add(to);
			if (!inDegrees.containsKey(to)) inDegrees.put(to, 1);
			else inDegrees.put(to, inDegrees.get(to));
//			if (!ins.containsKey(to)) ins.put(to, new HashSet<>());
//			ins.get(to).add(from);
		}
//		for (int k : graph.keySet()) {
//			if (!ins.containsKey(k)) ins.put(k, new HashSet<>());
//		}
		for (int k : graph.keySet()) {
			if (!inDegrees.containsKey(k)) inDegrees.put(k, 0);
		}

//		System.out.println(graph);
//		System.out.println(inDegrees);

		SLSimpleCycles solution = new SLSimpleCycles<>(graph, inDegrees);

		Set<Integer> commonNodes = solution.findSimpleCycles();
		boolean hasCycles = solution.hasCycles();

//		Johnson<Integer> solution = new Johnson<>();
//		List<Set<Integer>> cycles = solution.cycles(graph, ins);
//		Set<Integer> commonNodes = solution.getCommonNodes();
//		boolean hasCycles = cycles.size() > 0;

		if (hasCycles) {
			out.printLine(commonNodes.size());
			Object[] resultArray = commonNodes.stream().map(x -> x + 1).sorted().toArray();
			for (Object i : resultArray) {
				out.print(i);
				out.print(' ');
			}
		} else {
			out.printLine("NIE");
		}
		out.flush();
		out.close();
	}

}

class InputReader {

	private InputStream stream;
	private byte[] buf = new byte[1024];
	private int curChar;
	private int numChars;
	private SpaceCharFilter filter;

	public InputReader(InputStream stream) {
		this.stream = stream;
	}

	public int read() {
		if (numChars == -1)
			throw new InputMismatchException();
		if (curChar >= numChars) {
			curChar = 0;
			try {
				numChars = stream.read(buf);
			} catch (IOException e) {
				throw new InputMismatchException();
			}
			if (numChars <= 0)
				return -1;
		}
		return buf[curChar++];
	}

	public int readInt() {
		int c = read();
		while (isSpaceChar(c))
			c = read();
		int sgn = 1;
		if (c == '-') {
			sgn = -1;
			c = read();
		}
		int res = 0;
		do {
			if (c < '0' || c > '9')
				throw new InputMismatchException();
			res *= 10;
			res += c - '0';
			c = read();
		} while (!isSpaceChar(c));
		return res * sgn;
	}

	public String readString() {
		int c = read();
		while (isSpaceChar(c))
			c = read();
		StringBuilder res = new StringBuilder();
		do {
			res.appendCodePoint(c);
			c = read();
		} while (!isSpaceChar(c));
		return res.toString();
	}

	public boolean isSpaceChar(int c) {
		if (filter != null)
			return filter.isSpaceChar(c);
		return c == ' ' || c == '\n' || c == '\r' || c == '\t' || c == -1;
	}

	public String next() {
		return readString();
	}

	public interface SpaceCharFilter {
		public boolean isSpaceChar(int ch);
	}

}


class OutputWriter {

	private final PrintWriter writer;

	public OutputWriter(OutputStream outputStream) {
		writer = new PrintWriter(new BufferedWriter(new OutputStreamWriter(outputStream)));
	}

	public OutputWriter(Writer writer) {
		this.writer = new PrintWriter(writer);
	}

	public void print(Object... objects) {
		for (int i = 0; i < objects.length; i++) {
			if (i != 0)
				writer.print(' ');
			writer.print(objects[i]);
		}
	}

	public void printLine(Object... objects) {
		print(objects);
		writer.println();
	}

	public void close() {
		writer.close();
	}

	public void flush() {
		writer.flush();
	}

}

class SLSimpleCycles<V> {

	private Map<V, Collection<V>> graph;
	private Map<V, Integer> inDegrees;

	private List<List<V>> cycles = null;
	private V[] iToV = null;
	private Map<V, Integer> vToI = null;
	private Map<V, Set<V>> bSets = null;
	private ArrayDeque<V> stack = null;
	private Set<V> marked = null;
	private Map<V, Set<V>> removed = null;
	private int[] position = null;
	private boolean[] reach = null;
	private List<V> startVertices = null;

	private Set<V> commonNodes = null;

	public SLSimpleCycles(Map<V, Collection<V>> graph, Map<V, Integer> inDegrees) {
		if (graph == null) {
			throw new IllegalArgumentException("Null graph argument.");
		}
		this.graph = graph;
		this.inDegrees = inDegrees;
	}

	//	public List<List<V>> findSimpleCycles() {
	public Set<V> findSimpleCycles() {
		initState();
//		Kosaraju<V> kosaraju = new Kosaraju<>();
//		List<Set<V>> sccs = kosaraju.sccs(graph);
		SCCTarjan<V> tarjan = new SCCTarjan<>();
		List<Set<V>> sccs = tarjan.scc(graph);
//		if (sccs.size() > 1) return Collections.emptySet();
		for (Set<V> scc : sccs) {
			int maxInDegree = -1;
			V startVertex = null;
			for (V v : scc) {
				int inDegree = inDegrees.get(v);
				if (inDegree > maxInDegree) {
					maxInDegree = inDegree;
					startVertex = v;
				}
			}
			startVertices.add(startVertex);
		}

		for (V vertex : startVertices) {
			cycle(toI(vertex), 0);
			if (commonNodes.isEmpty()) break;
		}

//		List<List<V>> result = cycles;
//		return result;
		return commonNodes;
	}

	private boolean cycle(int v, int q) {
		boolean foundCycle = false;
		V vV = toV(v);
		marked.add(vV);
		stack.push(vV);
		int t = stack.size();
		position[v] = t;
		if (!reach[v]) {
			q = t;
		}
		Set<V> avRemoved = getRemoved(vV);
		Iterator<V> avIt = graph.get(vV).iterator();
		while (avIt.hasNext()) {
			V wV = avIt.next();
			if (avRemoved.contains(wV)) {
				continue;
			}
			int w = toI(wV);
			if (!marked.contains(wV)) {
				boolean gotCycle = cycle(w, q);
				if (gotCycle) {
					foundCycle = gotCycle;
				} else {
					noCycle(v, w);
				}
			} else if (position[w] <= q) {
				foundCycle = true;
				List<V> cycle = new ArrayList<V>();
				Iterator<V> it = stack.descendingIterator();
				V current = null;
				while (it.hasNext()) {
					current = it.next();
					if (wV.equals(current)) {
						break;
					}
				}
				cycle.add(wV);
				while (it.hasNext()) {
					current = it.next();
					cycle.add(current);
					if (current.equals(vV)) {
						break;
					}
				}
				cycles.add(cycle);
				commonNodes.retainAll(cycle);
				if (commonNodes.isEmpty()) return true;
			} else {
				noCycle(v, w);
			}
		}
		stack.pop();
		if (foundCycle) {
			unmark(v);
		}
		reach[v] = true;
		position[v] = graph.size();
		return foundCycle;
	}

	public boolean hasCycles() {
		return !cycles.isEmpty();
	}

	private void noCycle(int x, int y) {
		V xV = toV(x);
		V yV = toV(y);

		Set<V> by = getBSet(yV);
		Set<V> axRemoved = getRemoved(xV);

		by.add(xV);
		axRemoved.add(yV);
	}

	private void unmark(int x) {
		V xV = toV(x);
		marked.remove(xV);
		Set<V> bx = getBSet(xV);
		for (V yV : bx) {
			Set<V> ayRemoved = getRemoved(yV);
			ayRemoved.remove(xV);
			if (marked.contains(yV)) {
				unmark(toI(yV));
			}
		}
		bx.clear();
	}

	private void initState() {
		cycles = new ArrayList<List<V>>();
		iToV = (V[]) graph.keySet().toArray();
		vToI = new HashMap<V, Integer>();
		bSets = new HashMap<V, Set<V>>();
		stack = new ArrayDeque<V>();
		marked = new HashSet<V>();
		removed = new HashMap<V, Set<V>>();
		int size = graph.size();
		position = new int[size];
		reach = new boolean[size];
		startVertices = new ArrayList<V>();

		for (int i = 0; i < iToV.length; i++) {
			vToI.put(iToV[i], i);
		}

		commonNodes = new HashSet(graph.keySet());
	}

	private Integer toI(V v) {
		return vToI.get(v);
	}

	private V toV(int i) {
		return iToV[i];
	}

	private Set<V> getBSet(V v) {
		Set<V> result = bSets.get(v);
		if (result == null) {
			result = new HashSet<V>();
			bSets.put(v, result);
		}
		return result;
	}

	private Set<V> getRemoved(V v) {
		Set<V> result = removed.get(v);
		if (result == null) {
			result = new HashSet<V>();
			removed.put(v, result);
		}
		return result;
	}

}

class SCCTarjan<V> {

	Map<V, Collection<V>> graph;
	Map<V, Boolean> visited;
	Stack<V> stack;
	int time;
	Map<V, Integer> lowlink;
	List<Set<V>> components;

	public List<Set<V>> scc(Map<V, Collection<V>> graph) {
		this.graph = graph;
		visited = new HashMap<>();
		stack = new Stack<>();
		time = 0;
		lowlink = new HashMap<>();
		components = new ArrayList<>();

		for (V u : graph.keySet()) {
			lowlink.put(u, 0);
		}

		for (V u : graph.keySet())
			if (!visited.containsKey(u))
				dfs(u);

		return components;
	}

	void dfsRec(V u) {
		lowlink.put(u, time++);
		visited.put(u, true);
		stack.add(u);
		boolean isComponentRoot = true;

//		System.out.println("A---");
//		System.out.println(lowlink);
//		System.out.println(visited);
//		System.out.println(stack);
//		System.out.println(isComponentRoot);
//		System.out.println("---");

		for (V v : graph.get(u)) {
			if (!visited.containsKey(v))
				dfsRec(v);
			if (lowlink.get(u) > lowlink.get(v)) {
				lowlink.put(u, lowlink.get(v));
				isComponentRoot = false;
			}
		}

//		System.out.println("---");
//		System.out.println(lowlink);
//		System.out.println(visited);
//		System.out.println(stack);
//		System.out.println(isComponentRoot);
//		System.out.println("B---");

		if (isComponentRoot) {
			Set<V> component = new HashSet<>();
			while (true) {
				V x = stack.pop();
				component.add(x);
				lowlink.put(x, Integer.MAX_VALUE);
				if (x == u)
					break;
			}
			components.add(component);
		}
	}

	void dfs(V u) { dfsRec(u); }

	void dfsIt(V u) {
		boolean isComponentRoot = true;
		Stack<V> search = new Stack<>();
		search.push(u);
		while (!search.isEmpty()) {
			V u2 = search.pop();

			lowlink.put(u2, time++);
			visited.put(u2, true);
			stack.add(u2);

			System.out.println("---");
			System.out.println(lowlink);
			System.out.println(visited);
			System.out.println(stack);
			System.out.println(isComponentRoot);
			System.out.println("---");

			boolean wentSomewhere = false;

			for (V v : graph.get(u2)) {
				if (!visited.containsKey(v)) {
					wentSomewhere = true;
					search.push(v);
					continue;
				}
				if (lowlink.get(u2) > lowlink.get(v)) {
					lowlink.put(u2, lowlink.get(v));
					isComponentRoot = false;
				}
			}

			System.out.println("---");
			System.out.println(lowlink);
			System.out.println(visited);
			System.out.println(stack);
			System.out.println(isComponentRoot);
			System.out.println("---");

			if (!wentSomewhere)
			if (isComponentRoot) {
				Set<V> component = new HashSet<>();
				while (true) {
					V x = stack.pop();
					component.add(x);
					lowlink.put(x, Integer.MAX_VALUE);
					if (x == u2)
						break;
				}
				components.add(component);
			}
		}

	}

}

class Kosaraju<V> {


	public List<Set<V>> sccs(Map<V, Collection<V>> graph) {
		List<Set<V>> result = new ArrayList<>();
		Set<V> visited = new HashSet<>();
		Stack<V> s = new Stack<>();
		for (V v : graph.keySet()) {
			dfs(graph, visited, v, s);
		}
		Map<V, Collection<V>> transposed = transpose(graph);
//		System.out.println("transposed: " + transposed);
		visited = new HashSet<>();
		while (!s.isEmpty()) {
			V v = s.pop();
			if (!visited.contains(v)) {
				Set<V> comp = new HashSet<>();
				dfs(transposed, visited, v, comp);
				result.add(comp);
			}
		}
		return result;
	}

	void dfs(Map<V, Collection<V>> graph, Set<V> visited, V v, Stack<V> stack) {
		Stack<V> dfsStack = new Stack<>();
		if (!visited.contains(v)) {
			dfsStack.push(v);
			stack.push(v);
			while (!dfsStack.isEmpty()) {
				V popped = dfsStack.pop();
				visited.add(popped);
				for (V next : graph.get(popped)) {
					if (!visited.contains(next)) {
						dfsStack.push(next);
						stack.push(next);
					}
				}
			}
		}
	}

	void dfs(Map<V, Collection<V>> graph, Set<V> visited, V v, Set<V> stack) {
		Stack<V> dfsStack = new Stack<>();
		if (!visited.contains(v)) {
			dfsStack.push(v);
			stack.add(v);
			while (!dfsStack.isEmpty()) {
				V popped = dfsStack.pop();
				visited.add(popped);
				for (V next : graph.get(popped)) {
					if (!visited.contains(next)) {
						dfsStack.push(next);
						stack.add(next);
					}
				}
			}
		}
	}

	public Map<V, Collection<V>> transpose(Map<V, Collection<V>> graph) {
		Map<V, Collection<V>> transposed = new HashMap<>();
		for (V from : graph.keySet()) {
			if (!transposed.containsKey(from)) transposed.put(from, new HashSet<>());
			for (V to : graph.get(from)) {
				if (!transposed.containsKey(to)) transposed.put(to, new HashSet<>());
				transposed.get(to).add(from);
			}
		}
		return transposed;
	}

}

class Johnson<V> {

	class NodeAndNeighbors<V> {
		V node;
		Collection<V> neighbors;
		NodeAndNeighbors(V node, Collection<V> neighbors) {
			this.node = node;
			this.neighbors = neighbors;
		}

		@Override
		public String toString() {
			return "[" + node + "," + neighbors + "]";
		}
	}
	Set<V> commonNodes;

	public List<Set<V>> cycles(Map<V, Collection<V>> graph, Map<V, Collection<V>> ins) {
		commonNodes = new HashSet<>(graph.keySet());
		Kosaraju<V> kosaraju = new Kosaraju<>();
		LinkedList<Set<V>> sccs = new LinkedList<>(kosaraju.sccs(graph));
//		System.out.println("sccs: " + sccs);
		List<Set<V>> cycles = new ArrayList<>();
		while (!sccs.isEmpty()) {
			Set<V> scc = sccs.pop();
			V start = scc.iterator().next();
			LinkedList<V> path = new LinkedList<>();
			path.add(start);
			Set<V> blocked = new HashSet<>();
			Set<V> closed = new HashSet<>();
			blocked.add(start);
			Map<V, Set<V>> B = new HashMap<>();
			Stack<NodeAndNeighbors<V>> stack = new Stack<>();
			stack.push(new NodeAndNeighbors<>(start, new HashSet(graph.get(start))));
			while (!stack.isEmpty()) {
				NodeAndNeighbors<V> nodeAndNeighbors = stack.peek();
				V thisNode = nodeAndNeighbors.node;
				Collection<V> neighbors = nodeAndNeighbors.neighbors;
				if (!neighbors.isEmpty()) {
					V next = neighbors.iterator().next();
					neighbors.remove(next);
					if (next.equals(start)) {
						Set<V> cycle = new HashSet<>(path);
						cycles.add(cycle);
						commonNodes.retainAll(cycle);
						if (commonNodes.isEmpty()) return cycles;
						closed.addAll(path);
					} else if (!blocked.contains(next)) {
						path.add(next);
						stack.push(new NodeAndNeighbors<>(next, new HashSet(graph.get(next))));
						closed.remove(next);
						blocked.add(next);
						continue;
					}
				}
//				System.out.println("cycles: " + cycles);
//				System.out.println("graph: " + graph);
//				System.out.println("ins: " + ins);
//				System.out.println("start: " + start);
//				System.out.println("path: " + path);
//				System.out.println("blocked: " + blocked);
//				System.out.println("closed: " + closed);
//				System.out.println("B: " + B);
//				System.out.println("stack: " + stack);
//				System.out.println("neighbors: " + neighbors);
//				System.out.println("thisNode: " + thisNode);
				if (neighbors.isEmpty()) {
					if (closed.contains(thisNode)) {
//						System.out.println("unblock: " + thisNode + " " + blocked + " " + B);
						unblock(thisNode, blocked, B);
					} else {
						for (V neighbor : graph.get(thisNode)) {
							if (!B.containsKey(neighbor)) B.put(neighbor, new HashSet<>());
							if (!B.get(neighbor).contains(thisNode)) B.get(neighbor).add
									(thisNode);
						}
					}
					stack.pop();
					path.pop();
				}
			}
			removeNode(graph, ins, start);
			sccs.addAll(kosaraju.sccs(graph));
		}
		return cycles;
	}

	Set<V> getCommonNodes() {
		return commonNodes;
	}

	void removeNode(Map<V, Collection<V>> graph, Map<V, Collection<V>> ins, V node) {
//		System.out.println("TOREMOVE " + node);
//		System.out.println("! " + graph);
//		System.out.println("! " + ins);
		Set<V> removedNodePointingTo = new HashSet<>(graph.get(node));
		graph.remove(node);
		for (V v : ins.get(node)) {
			graph.get(v).remove(node);
//			if (graph.get(v).isEmpty()) graph.remove(v);
		}
		ins.remove(node);
		for (V pointingTo : removedNodePointingTo) {
			ins.get(pointingTo).remove(node);
//			if (ins.get(pointingTo).isEmpty()) ins.remove(pointingTo);
		}
//		System.out.println("! " + graph);
//		System.out.println("! " + ins);
	}

	Map<V, Collection<V>> subgraph(Map<V, Collection<V>> graph, Collection<V> nodesPersisted) {
		Map<V, Collection<V>> subgraph = new HashMap<>();
		Set<V> nodesToRemove = new HashSet<>(graph.keySet());
		nodesToRemove.removeAll(nodesPersisted);
		for (V node : nodesPersisted) {
			subgraph.put(node, new HashSet<>(graph.get(node)));
		}
		for (V node : subgraph.keySet()) {
			subgraph.get(node).removeAll(nodesToRemove);
		}
		return subgraph;
	}

	void unblock(V thisNode, Set<V> blocked, Map<V, Set<V>> B) {
		Set<V> stack = new HashSet<>();
		stack.add(thisNode);
		while (!stack.isEmpty()) {
			V node = stack.iterator().next();
			stack.remove(node);
			if (blocked.contains(node)) {
				blocked.remove(node);
				if (B.containsKey(node)) {
					stack.addAll(B.get(node));
					B.get(node).clear();
				}
			}
		}
	}
}