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import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.util.HashMap;
import java.util.HashSet;
import java.util.LinkedList;
import java.util.StringTokenizer;
import java.util.TreeSet;


public class kon {

	static HashMap<Integer, HashSet<Integer>> map = new HashMap<Integer, HashSet<Integer>>();
	static HashMap<Integer, HashSet<Integer>> mapT = new HashMap<Integer, HashSet<Integer>>();
	static HashMap<Integer, HashSet<Integer>> mapS = new HashMap<Integer, HashSet<Integer>>();
	static int n, m;
	static boolean[] visited;
	static int[] cycled;
	static int nr = 0;
	static int[] nrs;
	static int[] silaIn;
	static int[] silaOut;
	static HashMap<Integer, HashSet<Integer>> sss = new HashMap<Integer, HashSet<Integer>>();
	
	
	public static boolean cycle(int node, int ex) {
		// System.out.println("cycle(" + node + "," + ex);
		cycled[node] = 1;
		for(Integer neigh : mapS.get(node)) {
			// System.out.println("neigh" + neigh + " " + cycled[neigh]);
			if(neigh == ex) {
				continue;
			}
			if(cycled[neigh] == 1) {
				// System.out.println("true");
				return true;
			}
			if(cycled[neigh] == 0) {
				if(cycle(neigh, ex)) {
					return true;
				}
			}
		}
		cycled[node] = 2;
		return false;
	}

	public static void visit(int node) {
		visited[node] = true;
		for(Integer neigh : map.get(node)) {
			if(!visited[neigh]) {
				visit(neigh);
			}
		}
		// System.out.println(node);
		nrs[nr] = node;
		// System.out.println("post" + node);
		nr++;
	}
	
	public static void visit2(int node, int sssNum) {
		visited[node] = true;
		// System.out.println("node" + node);
		for(Integer neigh : mapT.get(node)) {
			if(!visited[neigh]) {
				visit2(neigh, sssNum);
			}
		}
		sss.get(sssNum).add(node);
	}
	
	
	public static void main(String[] args) throws IOException {
		BufferedReader reader = new BufferedReader(new InputStreamReader(System.in));
    	StringTokenizer tokenizer;
    	
    	tokenizer = new StringTokenizer(reader.readLine());
    	n = Integer.parseInt(tokenizer.nextToken());
    	m = Integer.parseInt(tokenizer.nextToken());
    	visited = new boolean[n];
    	nrs = new int[n];
    	silaIn = new int[n];
    	silaOut = new int[n];
    	cycled = new int[n];
    	for(int i = 0; i < n; i++) {
    		silaIn[i] = 0;
    		silaOut[i] = 0;
    		visited[i] = false;
    		map.put(i, new HashSet<Integer>());
    		mapT.put(i, new HashSet<Integer>());
    	}

    	for(int i = 0; i < m; i++) {
    		tokenizer = new StringTokenizer(reader.readLine());
    		int a = Integer.parseInt(tokenizer.nextToken());
    		int b = Integer.parseInt(tokenizer.nextToken());
    		map.get(a-1).add(b-1);
    		mapT.get(b-1).add(a-1);
    	}
    	
    	for(int i = 0; i < n; i++) {
    		if(!visited[i]) {
    			visit(i);
    		}
    	}
    	
    	for(int i = 0; i < n; i++) visited[i] = false;
    	
    	int sssCnt = 0;
    	for(int i = 0; i < n; i++) {
    		if(!visited[nrs[n-1-i]]) {
    			sss.put(sssCnt, new HashSet<Integer>());
    			visit2(nrs[n-1-i], sssCnt);
    			sssCnt++;
    		}
    	}
    	
    	HashSet<Integer> s = null;
    	for(HashSet<Integer> ss : sss.values()) {
    		if(ss.size() > 1) {
    			if(s == null) {
    				s = ss;
    			} else {
    				System.out.println("NIE");
                    System.exit(0);
    			}
    		}
    	}
    	
    	if(s == null) {
    		System.out.println("NIE");
            System.exit(0);
    	}
    	
    	
    	int silaO = 0;
    	int silaI = 0;
    	for(Integer node : s) {
    		// silaOut[node] = map.get(node).size();
    		for(Integer neigh : map.get(node)) {
    			if(s.contains(neigh)) {
    				silaOut[node]++;
    				silaO++;
	    			silaIn[neigh]++;
	    			silaI++;
    			}
    		}
    		// System.out.println("s elem " + node + "  so=" + silaOut[node] + "  si=" + silaIn[node]);
    	}
    	// System.out.println(s.size() + "   " + silaO + "  " + silaI);
    	
    	
    	for(Integer node : s) {
    		mapS.put(node, new HashSet<Integer>());
    	}
    	for(Integer node : s) {
    		for(Integer neigh : map.get(node)) {
    			if(s.contains(neigh)) {
    				mapS.get(node).add(neigh);
    			}
    		}
    	}

    	HashSet<Integer> ret = new HashSet<Integer>();
    	int cnt = 0;
    	for(Integer node : s) {	// TODO: zbyt brutalne
    		// System.out.println((cnt++) + "/" + s.size()); System.out.flush();
    		
    		if(silaO - silaOut[node] - silaIn[node] <= s.size()-1-1) {
    			ret.add(node);
    		} else {
    			for(Integer i : s) cycled[i] = 0;
    			boolean cycle = false;
    			for(Integer i : s) {
    				// System.out.print("   " + i);
    				if(i == node) {
    					continue;
    				}
    				if(cycled[i] == 0) {
    					if(cycle(i, node)) {
    						// System.out.println("cycle" + i + "," + node);
    						cycle = true;
    						// System.out.println("breakinf on node " + node);
        					break;
        				}
    				}
    			}
    			if(!cycle) {
    				// System.out.println(" no cycle without " + node);
    				ret.add(node);
    			}
    		}
    		// for(Integer i : s) visited[i] = false;
    		//if(!cycle(node)) {
    		//	ret.add(node);
    		//}
    	}
    	
    	System.out.println(ret.size());
    	StringBuilder sb = new StringBuilder();
    	boolean emptySb = true;
    	for(Integer cross : ret) {
    		if(!emptySb) {
    			sb.append(" ");
    		}
    		sb.append(cross+1);
    		emptySb = false;
    	}
    	System.out.println(sb.toString());
    	System.exit(0);
    }
}