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import java.io.BufferedInputStream;
import java.io.BufferedOutputStream;
import java.io.InputStream;
import java.io.OutputStream;
import java.util.ArrayList;
import java.util.Comparator;
import java.util.HashSet;
import java.util.Iterator;
import java.util.LinkedList;
import java.util.List;

public class kon {
	
	static Vertex[] vertices;
	static int n, m;
	
	static class Vertex {
		public Vertex(int nr) {
			this.nr = nr;
		}
		@Override
		public String toString() {
			return "" + nr;
		}
		@Override
		public int hashCode() {
			return nr;
		}
		int nr;
		LinkedList<Vertex> next = new LinkedList<Vertex>();
		LinkedList<Vertex> prev = new LinkedList<Vertex>();
		
		int realNext = 0;
		int realPrev = 0;
		boolean dummy = false;
		int state;
		boolean sure = false;
		boolean sureNot = false;
	}
	
	public static void main(String[] args) {
		n = R.i();
		m = R.i();
		vertices = new Vertex[n + 1];
		for (int i = 1; i <= n; i++) {
			vertices[i] = new Vertex(i);
		}
		int a, b;
		for (int i = 0; i < m; i++) {
			a = R.i();
			b = R.i();
			vertices[a].next.add(vertices[b]);
			vertices[a].realNext++;
			vertices[b].prev.add(vertices[a]);
			vertices[b].realPrev++;
		}
		
		LinkedList<Vertex> dummies = new LinkedList<>();
		for (int i = 1; i <= n; i++) {
			if (vertices[i].realNext == 0 || vertices[i].realPrev == 0) {
				dummies.add(vertices[i]);
			}
		}
		
		Vertex d;
		while (!dummies.isEmpty()) {
			d = dummies.poll();
			if (!d.dummy) {
				d.dummy = true;
				for (Vertex prev : d.prev) {
					prev.realNext--;
					if (prev.realNext == 0) {
						dummies.add(prev);
					}
				}
				for (Vertex next : d.next) {
					next.realPrev--;
					if (next.realPrev == 0) {
						dummies.add(next);
					}
				}
			}
		}
		
		Vertex vv;
		for (int i = 1; i <= n; i++) {
			d = vertices[i];
			for (Iterator<Vertex> iter = d.next.iterator(); iter.hasNext(); ) {
				vv = iter.next();
				if (vv.dummy) iter.remove();
			}
			for (Iterator<Vertex> iter = d.prev.iterator(); iter.hasNext(); ) {
				vv = iter.next();
				if (vv.dummy) iter.remove();
			}
		}
		
		LinkedList<Vertex> c = fc(null);
		if (c == null) {
			W.sn("NIE");
		} else {
			LinkedList<Vertex> candidates = c;
			HashSet<Vertex> newCandidates = new HashSet<Vertex>(n);
			newCandidates.addAll(c);
			Vertex v;
			for (int i = 1; i <= n; i++) {
				v = vertices[i];
				if (!v.dummy && !newCandidates.contains(v)) {
					v.sureNot = true;
				}
			}
			ArrayList<Integer> res = new ArrayList<Integer>();
			while (!candidates.isEmpty()) {
				//System.out.println(candidates.size());
				Vertex candidate = candidates.poll();

				boolean checked = false;
				if (candidate.prev.size() == 1 && candidate.prev.peek().sureNot) {
					candidate.sureNot = true;
					checked = true;
				}

				if (!checked) {
					for (Vertex prev : candidate.prev) {
						if (prev.sure && prev.next.size() == 1) {
							res.add(candidate.nr);
							candidate.sure = true;
							checked = true;
						}
					}
				}
				
				if (!checked) {
					//System.out.println("fire");
					c = fc(candidate);
					if (c == null) {
						res.add(candidate.nr);
						candidate.sure = true;
					} else {
						newCandidates.clear();
						newCandidates.addAll(c);
						candidate.sureNot = true;
						for (Iterator<Vertex> iter = candidates.iterator(); iter.hasNext();) {
							v = iter.next();
							if (!newCandidates.contains(v)) {
								v.sureNot = true;
								iter.remove();
							}
						}
					}
				}
			}
			W.in(res.size());
			if (res.size() > 0) {
				res.sort(new Comparator<Integer>() {
					@Override
					public int compare(Integer arg0, Integer arg1) {
						return arg0 - arg1;
					}
				});
				for (Integer i : res) {
					W.is(i);
				}
			}
		}
		
		W.flush();
	}

	static class Q {
		public Q(Vertex v, int l) {
			this.v = v;
			this.l = l;
		}
		Vertex v;
		int l;
	}
	
	static LinkedList<Vertex> fc(Vertex skip) {
		LinkedList<Vertex> pp = new LinkedList<Vertex>();
		
		for (int i = 1; i <= n; i++) {
			vertices[i].state = 0;
		}
		
		LinkedList<Q> queue = new LinkedList<Q>();
		Q q;
		Vertex v, vv;
		for (int i = 1; i <= n; i++) {
			vv = vertices[i];
			if (vv != skip && !vv.dummy && vertices[i].state == 0) {
				queue.push(new Q(vertices[i], 0));
				while (!queue.isEmpty()) {
					q = queue.poll();
					
					while (pp.size() > q.l) {
						pp.getLast().state = 2;
						pp.removeLast();
					};

					v = q.v;
					
					if (v.state == 0) {
						pp.add(v);
						v.state = 1;
	
						for (Vertex e: v.next) {
							if (e != skip) {
								if (e.state == 1) {
									LinkedList<Vertex> result = new LinkedList<Vertex>();
									Iterator<Vertex> iter = pp.descendingIterator();
									Vertex x;
									while ((x = iter.next()) != e) {
										result.push(x);
									}
									result.push(e);
									return result;
								} else if (e.state == 0) {
									queue.push(new Q(e, q.l + 1));
								}
							}
						}
					}
				}
			}
		}
		
		return null;
	}
	

	static class R {
		static InputStream is=new BufferedInputStream(System.in);
		static int b='\n';
		static void next(){try{b=is.read();}catch(Exception e){}}
		static void skip(){while(b=='\n'||b==' '||b=='\r'||b=='\t')next();}
		static char c(){skip();char c=(char)b;next();return c;}
		static int i(){skip();int i=0;while(b>='0'&&b<='9'){i=10*i+(b-'0');next();}return i;}
		static long l(){skip();long i=0;while(b>='0'&&b<='9'){i=10*i+(b-'0');next();}return i;}
	}
	static class W {
		static OutputStream os=new BufferedOutputStream(System.out,4096);
		static void b(int b){try{os.write(b);}catch(Exception e){}}
		static void c(char c){b(c);}
		static void cs(char c){c(c);space();}
		static void cn(char c){c(c);newLine();}
		static void i(int i){if(i>9)i(i/10);b('0'+(i%10));}
		static void is(int i){i(i);space();}
		static void in(int i){i(i);newLine();}
		static void l(long l){if(l>9)l(l/10);b('0'+(int)(l%10));}
		static void ls(long l){l(l);space();}
		static void ln(long l){l(l);newLine();}
		static void s(String s){try{os.write(s.getBytes());}catch(Exception e){}}
		static void ss(String s){s(s);space();}
		static void sn(String s){s(s);newLine();}
		static void tn(boolean b){sn(b?"TAK":"NIE");}
		static void space(){b(' ');}
		static void newLine(){b('\n');}
		static void flush(){try{os.flush();}catch(Exception e){}};
	}
	static class T {
		static List<String> s=new ArrayList<String>();
		static List<Long> t=new ArrayList<Long>();
		static void t(String n){s.add(n);t.add(System.currentTimeMillis());}
		static void err(){t.add(System.currentTimeMillis());System.err.println();for(int i=0;i<s.size();i++){System.err.println(s.get(i)+": "+(t.get(i+1)-t.get(i)));}System.err.println("TOTAL: "+(t.get(t.size() - 1)-t.get(0)));System.err.flush();}
	}

}