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import java.util.ArrayList;
import java.util.Iterator;
import java.util.Scanner;


public class kon {

	static Node[] nodes;

	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		int n = sc.nextInt(), m = sc.nextInt();

		nodes = new Node[n + 1];
		for (int i = 0; i < m; i++) {
			int a = sc.nextInt(), b = sc.nextInt();
			Node nodeA = getOrCreate(a);
			Node nodeB = getOrCreate(b);
			nodeA.out.add(nodeB);
			nodeB.in.add(nodeA);
		}

		// Optimize graph
		while (true) {
			int counter = 0;
			for (int i = 1; i <= n; i++) {
				Node node = nodes[i];
				if (node != null && (node.out.isEmpty() || node.in.isEmpty())) {
					counter += removeNode(node);
				}
			}
			if (counter == 0) {
				break;
			}
		}

		/*for (int i = 1; i <= n; i++) {
			Node node = nodes[i];
			if (node != null && !node.dead) {
				System.out.println("Node: " + node + " Out: " + node.out + " In: " + node.in);
			}
		}*/

		ArrayList<ArrayList<Node>> total = new ArrayList<ArrayList<Node>>();
		for (int i = 1; i <= n; i++) {
			Node node = nodes[i];
			if (node != null && !node.dead) {
				ArrayList<Node> initPath = new ArrayList<Node>();
				initPath.add(node);
				dfs(node, node, initPath, total);
			}
		}

		if (total.size() == 0) {
			System.out.println("NIE");
			return;
		}

		int expectedCount = total.size();
		int[] nodesCount = new int[n + 1];
		for (ArrayList<Node> path : total) {
			for (Node node : path) {
				nodesCount[node.n] += 1;
			}
		}

		StringBuilder sb = new StringBuilder(32);
		int idx = 0;
		for (int i = 1; i <= n; i++) {
			if (nodesCount[i] == expectedCount) {
				idx += 1;
				sb.append(nodes[i]).append(' ');
			}
		}
		System.out.println(idx);
		System.out.println(sb);
	}

	static void dfs(Node node, Node initNode, ArrayList<Node> path, ArrayList<ArrayList<Node>> total) {
		for (Node outNode : node.out) {
			if (outNode.n == initNode.n) {
				total.add(path);
			} else if (!path.contains(outNode)) {
				ArrayList<Node> newPath = new ArrayList<Node>(path);
				newPath.add(outNode);
				dfs(outNode, initNode, newPath, total);
			}
		}
	}

	static int removeNode(Node node) {
		int counter = 0;
		Iterator<Node> outIterator = node.out.iterator();
		while (outIterator.hasNext()) {
			Node n = outIterator.next();
			if (n.in.remove(node)) {
				counter += 1;
			}
			outIterator.remove();
		}
		Iterator<Node> inIterator = node.in.iterator();
		while (inIterator.hasNext()) {
			Node n = inIterator.next();
			if (n.out.remove(node)) {
				counter += 1;
			}
			inIterator.remove();
		}
		node.dead = true;
		return counter;
	}

	static Node getOrCreate(int n) {
		if (nodes[n] == null) {
			nodes[n] = new Node(n);
		}
		return nodes[n];
	}

	static class Node {
		private int n;
		private boolean dead;
		private ArrayList<Node> out = new ArrayList<Node>();
		private ArrayList<Node> in = new ArrayList<Node>();

		Node(int n) {
			this.n = n;
		}

		@Override
		public boolean equals(Object obj) {
			return n == ((Node) obj).n;
		}

		@Override
		public int hashCode() {
			return Integer.valueOf(n).hashCode();
		}

		@Override
		public String toString() {
			return "" + n;
		}
	}
}