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#include <cstdio>
#include <iostream>
#include <algorithm>
#include <string>
#include <vector>

using namespace std;

typedef vector<int> VI;
typedef long long LL;

/* PA-2015 R5 */
/* Korzystałem z kodu zaczęrpniętego z książki Piotr Stańczyka "Algorytmika Praktyczna" - topo sort */

#define FOR(x, b, e) for(int x = b; x <= (e); ++x)
#define FORD(x, b, e) for(int x = b; x >= (e); --x)
#define REP(x, n) for(int x = 0; x < (n); ++x)
#define VAR(v, n) __typeof(n) v = (n)
#define ALL(c) (c).begin(), (c).end()
#define SIZE(x) ((int)(x).size())
#define FOREACH(i, c) for(VAR(i, (c).begin()); i != (c).end(); ++i)
#define PB push_back
#define ST first
#define ND second
template<class V, class E> struct Graph {
	struct Ed : E {
		int v; 
		Ed(E p, int w) : E(p), v(w) {}
	};
	struct Ve : V,vector<Ed> {};
	vector<Ve> g;
	Graph(int n=0) : g(n) {}
	void EdgeD(int b, int e, E d = E()) {g[b].PB(Ed(d,e));}
  int topo;
  void TopoDfs(int v){
	  if (!g[v].t) {
		g[v].t=1; 
		FOREACH(it,g[v]) 
		{
			TopoDfs(it->v);
		}
		g[v].t=--topo;
	  }
  }
  void TopoSort(){
    FOREACH(it,g) it->t=0; topo=SIZE(g);
	FORD(x,topo-1,0) TopoDfs(x);
  }
  VI TopoSortV(){
    VI res(SIZE(g));
    TopoSort();


    REP(x,SIZE(g)) res[g[x].t] = x;
    return res;
  }

};
struct Ve {}; 
struct Vs {
	int t;
};
int main() {
	int n, m, b, e;
	cin >> n >> m;
	Graph<Vs, Ve> g(n);
	REP(x,m) {
		cin >> b >> e;
		g.EdgeD(b - 1, e - 1);
	}
	VI res = g.TopoSortV();


	vector<int> inter;
	bool joined = false;

	bool cycle_found = false;	
	FOREACH(it, g.g) 
		FOREACH(it2, *it) 
			if (it->t >= g.g[it2->v].t) 
			{
					cycle_found = true;
	//				cout<< it - g.g.begin() + 1 << " " << it2->v +1 << endl;
					int from = it - g.g.begin();
					int to = it2->v;

					int topos = g.g[to].t;
					int frompos = g.g[from].t;

//					cout << topos << " " << frompos << endl;
										
					vector<int> out (res.begin() + topos, res.begin() + frompos + 1);
					std::sort(out.begin(), out.end());
			
					if(!joined)
					{
						inter.insert(inter.begin(), out.begin(), out.end());
						joined = true;
					}
					else
					{
							vector<int> tmp(inter.begin(), inter.end());
							inter.clear();
					
						  std::set_intersection(out.begin(), out.end(),
                          tmp.begin(), tmp.end(),
                          std::back_inserter(inter));

					}
					
	

			}

	if(!cycle_found)
	{
		cout << "NIE\n";
	}
	else
	{

		cout << inter.size() << endl;
		if(inter.size())
		{
			FOREACH(it, inter) cout << *it  + 1<< " ";
			cout <<endl;
		}
	}


	return 0;
}