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#include <cstdlib>
#include <set>
#include <stack>
#include <vector>
#include <iostream>

using namespace std;


typedef vector<int> Vertex;
typedef vector<Vertex> Graph;


void strongConnect(Graph const &gr, vector<int> &idx, vector<int> &low, stack<int> &st, vector<bool> &on, int &index, int const &v, vector< vector<int> > &scs) {
  idx[v] = index;
  low[v] = index;
  ++index;
  
  st.push(v);
  on[v] = true;
  
  for(unsigned int i = 0; i < gr[v].size(); ++i) {
    int w = gr[v][i];
    
    if(idx[w] == -1) {
      strongConnect(gr, idx, low, st, on, index, w, scs);
      low[v] = min(low[v], low[w]);
    } else if(on[w]) {
      low[v] = min(low[v], idx[w]);
    }
  }
  
  if(idx[v] == low[v]) {
    scs.push_back(vector<int>());
    while(true) {
      int w = st.top(); st.pop();
      on[w] = false;
      scs.back().push_back(w);
      if(w == v) break;
    }
    if(scs.back().size() == 1) scs.pop_back();
  }
}


int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  
  srand(0xdeadbeef);
  
  int n, m;
  cin >> n >> m;
  
  Graph gr(n);
  
  for(int i = 0; i < m; ++i) {
    int a, b;
    cin >> a >> b; --a; --b;
    gr[a].push_back(b);
  }
  
  int index = 0;
  vector<int> idx(n, -1);
  vector<int> low(n, -1);
  vector<bool> on(n, false);
  stack<int> st;
  vector< vector<int> > scs;
  
  for(int v = 0; v < n; ++v) {
    if(idx[v] == -1) {
      strongConnect(gr, idx, low, st, on, index, v, scs);
    }
  }
  
  if(scs.size() == 0) {
    cout << "NIE\n";
    return 0;
  }
  
  if(scs.size() > 1) {
    cout << "0\n";
    return 0;
  }
  
  vector<int> &osc = scs[0];
  set<int> oscs;
  for(unsigned int i = 0; i < osc.size(); ++i) oscs.insert(osc[i]);
  
  for(unsigned int i = 0; i < osc.size(); ++i) {
    int v = osc[i];
    for(unsigned int j = 0; j < gr[v].size(); ++j) {
      int w = gr[v][j];
      if(oscs.find(w) == oscs.end()) {
        gr[v][j] = gr[v].back();
        gr[v].pop_back();
        --j;
      }
    }
  }
  
  int stepsLeft = 2000000;
  for(int i = 0; stepsLeft > 0; ++i) {
    int v0 = osc[rand() % osc.size()];
    int offset = rand();
    
    stack<int> st;
    stack<int> ft;
    st.push(v0);
    ft.push(v0);
    on[v0] = true;
    
    while(not st.empty()) {
      int v = st.top(); st.pop();
      --stepsLeft;
      bool broken = false;
      for(unsigned int j = 0; j < gr[v].size(); ++j) {
        int w = gr[v][(j + offset) % gr[v].size()];
        if(w == v0) {
          for(set<int>::iterator it = oscs.begin(); it != oscs.end();) {
            if(not on[*it]) {
              oscs.erase(it++);
            } else {
              ++it;
            }
          }
          broken = true;
          break;
        }
      }
      if(broken) break;
      for(unsigned int j = 0; j < gr[v].size(); ++j) {
        int w = gr[v][(j + offset) % gr[v].size()];
        if(not on[w]) {
          st.push(w);
          ft.push(w);
          on[w] = true;
          break;
        }
      }
    }
    
    while(not ft.empty()) { on[ft.top()] = false; ft.pop(); }
  }
  
  cout << oscs.size() << '\n';
  for(set<int>::iterator it = oscs.begin(); it != oscs.end(); ++it) {
    cout << 1 + *it << ' ';
  } cout << '\n';
  
  return 0;
}