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#include <iomanip>
#include <iostream>
#include <queue>
#include <utility>
#include <algorithm>
#include <cassert>
#include <string>
#include <vector>
#include <set>
#include <map>
#include <cstdlib>
#include <unordered_map>

using namespace std;

#define ALL(x) x.begin(), x.end()
#define VAR(a,b) __typeof (b) a = b
#define REP(i,n) for (int _n=(n), i=0; i<_n; ++i)
#define FOR(i,a,b) for (int _b=(b), i=(a); i<=_b; ++i)
#define FORD(i,a,b) for (int _b=(b), i=(a); i>=_b; --i)
#define FORE(i,a) for (VAR(i,a.begin ()); i!=a.end (); ++i) 
#define IN(x) int x; cin >> x
#define PB push_back
#define MP make_pair
#define ST first
#define ND second

typedef vector<int> VI;
typedef long long LL;
typedef pair<int,int> PII;
typedef double LD;

const int MAXN = 5 * int(1e5);

int n;

VI on_stack;

VI stack;

VI v[MAXN];

VI is_disabled, vis;

VI any_cycle;

bool found_cycle;

void dfs(int node) {
  on_stack[node] = 1;
  vis[node] = 1;
  stack.PB(node);
  FORE(it,v[node])
    if (found_cycle) break;
    else if (is_disabled[*it]) continue;
    else if (!vis[*it]) dfs(*it);
    else {
      if (!on_stack[*it]) continue;
      found_cycle = true;
      if (any_cycle.empty()) {
        bool found_first = false;
        FORE(jt, stack) {
          if (!found_first && *jt != *it) continue;
          found_first = true;
          any_cycle.PB(*jt);
        }
      }
    }
  on_stack[node] = 0;
  stack.pop_back();
}

int main() {
  ios_base::sync_with_stdio(0);
  cout.setf(ios::fixed);
  int m;
  cin >> n >> m;
  REP(i,m) {
    IN(a); IN(b);
    --a; --b;
    v[a].PB(b);
  }
  REP(i,n)
    random_shuffle(ALL(v[i]));
  on_stack = is_disabled = vis = VI(n, 0);
  found_cycle = false;
  VI to_try;
  REP(i,n)
    to_try.PB(i);
  random_shuffle(ALL(to_try));
  FORE(it, to_try)
    if (any_cycle.empty() && !vis[*it])
      dfs(*it);
  if (!found_cycle) {
    cout << "NIE\n";
    return 0;
  }
  VI res;
  FORE(it, any_cycle) {
    found_cycle = false;
    is_disabled[*it] = 1;
    vis = VI(n, 0);
    REP(i, n)
      if (!found_cycle && !vis[i])
        dfs(i);
    is_disabled[*it] = 0;
    if (!found_cycle) res.PB(*it);
  }
  sort(ALL(res));
  cout << res.size() << endl;
  if (res.empty()) return 0;
  FORE(it, res) cout << *it + 1 << " ";
  cout << endl;
  return 0;
}