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#include <cstdio>
#include <iostream>
#include <algorithm>
#include <string>
#include <vector>
//#include <complex>
//#include <iterator>
#include <set>
//#include <bitset>
#include <map>
//#include <stack>
#include <list>
//#include <queue>
//#include <deque>
using namespace std;
typedef vector<int> VI;
typedef long long LL;
#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(i= (c).begin(); i !=(c).end(); ++i)
#define FORE(i, c) for(VAR(i, (c).begin()), KS=(c).end(); i !=KS; ++i)
#define PB push_back
#define ST first
#define ND second
const int INF = 1000000001;
const double EPS = 10e-9;
typedef vector<VI> VVI;
typedef vector<LL> VLL;
typedef vector<double> VD;
typedef vector<string> VS;
typedef pair<int, int> PII;
typedef vector<PII> VPII;
#define PF push_front
#define MP make_pair
struct tri
{
  int a,b,c;
  tri(int k=0,int l=0,int m=0)
  {
    a=k;
    b=l;
    c=m;
  }
};
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));
  }
  Ve tym;
/*  void Write() {
    typename vector<Ed>::iterator it;
    REP(x, SIZE(g)) {
      cout << x << ":";
      FOREACH(it, g[x]) cout << " " << it->v;
      cout << endl;
    }
  }*/
  void EdgeU(int b, int e, E d = E()) {
    Ed eg(d, e);
    eg.rev = SIZE(g[e]) + (b == e);
    g[b].PB(eg);
    eg.rev = SIZE(g[eg.v = b]) - 1;
    g[e].PB(eg);
  }
   int topo;
   void TopoDfs(int v) {
     if (!g[v].t) {
       g[v].t = 1;
       typename vector<Ed>::iterator it;
       FOREACH(it, g[v]) TopoDfs(it->v);
       g[v].t = --topo;
     }
   }
   void TopoSort() {
     typename vector<Ve>::iterator it;
     FOREACH(it, g) it->t = 0;
     topo = SIZE(g);
     FORD(x, topo - 1, 0) TopoDfs(x);
   }
  bool AcyclicD() {
    TopoSort();
  typename vector<Ve>::iterator it;
  typename vector<Ed>::iterator it2;
  FOREACH(it, g) FOREACH(it2, *it) if (it->t >= g[it2->v].t) return false;
  return true;
  }
};
struct vv
{
  int t;
};
struct ee
{

};
int main()
{
  int i,n,m,a,b;
  scanf("%d%d",&n,&m);
  Graph<vv,ee> gr(n+1);
  VI wyn;
  for(i=0;i<m;++i)
  {
    scanf("%d%d",&a,&b);
    gr.EdgeD(a,b);
  }
  if(gr.AcyclicD())
  {
    printf("NIE\n");
    return 0;
  }
  for(i=1;i<=n;++i)
  {
    gr.tym=gr.g[i];
    gr.g[i].clear();
    if(gr.AcyclicD())
      wyn.PB(i);
    gr.g[i]=gr.tym;
  }
  printf("%d\n",wyn.size());
  for(i=0;i<SIZE(wyn);++i)
    printf("%d ",wyn[i]);
  return 0;
}