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#include <cstdio>
#include <iostream>
#include <algorithm>
#include <cstring>
#include <vector>
#include <set>
#include <map>
#include <cmath>
#include <list>
#include <ctime>
#include <sstream>
#include <queue>
#include <stack>
#include <bitset>
#include <unordered_set>
#include <unordered_map>
using namespace std;
typedef vector<int> vi;
typedef pair<int,int> pii;
typedef long long ll;
typedef short int sint;
#define FOR(x, b, e) for(int x=(b); x<=(e); ++x)
#define FORD(x, b, e) for(int x=((int)(b))-1; x>=(e); --x)
#define REP(x, n) for(int x=0; x<(n); ++x)
#define ALL(c) c.begin(),c.end()
#define SIZE(x) ((int)((x).size()))
#define PB push_back
#define ST first
#define ND second
#define mp(x,y) make_pair(x,y)
#define DEBUG 1
#define debug(x) {if (DEBUG)cerr <<#x <<" = " <<x <<endl; }
#define debugv(x) {if (DEBUG) {cerr <<#x <<" = "; FOREACH(it, (x)) cerr <<*it <<", "; cout <<endl; }}
#define REMAX(a,b) (a)=max((a),(b));
#define REMIN(a,b) (a)=min((a),(b));
#define wez(n) int (n); scanf("%d",&(n));
#define wez2(n,m) int (n),(m); scanf("%d %d",&(n),&(m));

bool dbg = false;

const int N = 501001;
vi za[N];
int ter[N];
int stos[N], ind;
stack<int> S;
int n, m, d;
int color[N];
int dt[N];
bool cycleDetected, onCycle[N];
set<pii> onStack;
set<pii> notOnStack;

int Stop() {
  return stos[ind - 1];
}

void Spop() {
  --ind;
}
void Spush(int x) {
  stos[ind++] = x;
}
bool Sempty() {
  return ind == 0;
}

void visit(int x) {
  Spush(x);
  while (!Sempty()) {
    int curr = Stop();
    if (dbg) printf("in: %d\n", curr);
    if (ter[curr] >= SIZE(za[curr])) {
      // skonczylem curr
      if (dbg) printf("\tfinished\n");
      Spop();
      color[curr] = 2;
      auto it = onStack.find(mp(dt[curr], curr));
      if (it != onStack.end()) {
        onStack.erase(it);
        notOnStack.insert(mp(dt[curr], curr));
      }
      continue;
    }
    if (color[curr] == 0) {
      dt[curr] = ++d;
      color[curr] = 1;
      if (dbg) printf("\tfirst: %d\n", curr);
    }
    int nast = za[curr][ter[curr]++];
    if (color[nast] == 0) {
      Spush(nast);
      if (dbg) printf("\tnew edge: %d->%d\n", curr, nast);
    } else if (color[nast] == 1) {
      // cycle detected
      if (dbg) printf("\tcycle with: %d->%d\n", curr, nast);
      if (cycleDetected) {
        onCycle[curr] = onCycle[nast] = true;
        notOnStack.clear();
        auto c = onStack.begin();
        while (c != onStack.end() && dt[nast] > c->ST) {
          onStack.erase(c);
          c = onStack.begin();
        }
      } else {
        // wrzuc kandydatow
        int x = ind - 1;
        while (dt[stos[x]] >= dt[nast]) {
          onStack.insert(mp(dt[stos[x]], stos[x]));
          onCycle[stos[x]] = true;
          --x;
        }
        cycleDetected = true;
      }
    } else { // color[nast] == 2
      // forward edge detected
      if (!onCycle[nast]) {
        continue;
      }
      if (dbg) printf("\tforward: %d->%d\n", curr, nast);
      auto maxOnStack = onStack.rbegin();
      if (maxOnStack != onStack.rend()) {
        while (true) {
        auto c = notOnStack.lower_bound(*maxOnStack);
        if (c != notOnStack.end() && c->ST < dt[nast]) {
          notOnStack.erase(c);
        } else {
          break;
        }
      }
      }
    }
  }
}

int main() {
  scanf("%d %d", &n, &m);
  REP(i, m) {
    wez2(a, b);
    za[a].PB(b);
  }
  FOR(i, 1, n) {
    if (color[i] == 0) {
      visit(i);
    }
  }
  if (!cycleDetected) {
    printf("NIE\n");
  } else {
    vi poss(SIZE(onStack) + SIZE(notOnStack));
    int xx = 0;
    for (auto elem : onStack) {
      poss[xx++] = elem.ND;
    }
    for (auto elem : notOnStack) {
      poss[xx++] = elem.ND;
    }
    sort(ALL(poss));
    printf("%d\n", SIZE(poss));
    if (!poss.empty()) {
      for (auto elem : poss) {
        printf("%d ", elem);
      }
      printf("\n");
    }
  }
  return 0;
}