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#include <iostream>
#include <vector>
#include <cstdio>

using namespace std;

int N, M;

vector< vector<int> > edges(1000001, vector<int>());

int half = 500001;
int visited[1000002];
bool res_nodes[500002];

int NOT_VISITED = 0;
int PATH = 1;
int VISITED = 2;

int dfs(int nr) {
  if (visited[nr] == PATH) return 1;
  if (visited[nr] == VISITED) return 0;
  
  visited[nr] = PATH;
  int sum = 0;
  for(std::vector<int>::iterator it = edges[nr].begin(); it != edges[nr].end(); ++it) {
    sum += dfs(*it);
  }
  visited[nr] = VISITED;
  
  if(sum > 0) return 1;
  return 0;
}
pair<int, int> count_cycles() {
  int cycles = 0;
  int st = 0;
  for(int i = 1; i <= N; i++){
    visited[i] = NOT_VISITED; visited[i + half] = NOT_VISITED;
  }
  for(int i = 1; i < N; i++) {
    if(visited[i] == NOT_VISITED) {
      int d = dfs(i);
      if (d > 0 && st == 0) st = i;
      cycles += d;
    }
  }
  
  return std::make_pair(cycles, st);
}

int main() {

  scanf("%d %d", &N, &M);
  
  int begin, end;
  
  for(int i = 1; i <= M; i++){
    scanf("%d %d", &begin, &end);
    
    edges[begin + half].push_back(end);
  }
  for(int i = 1; i <= N; i++)edges[i].push_back(half + i);
  
  pair<int, int> res = count_cycles();
  int independent_cycles = res.first;
  int node = res.second;
  int final_nodes = 0;
  if (independent_cycles == 0){
    printf("NIE\n");
  } else if (independent_cycles > 1) {
    printf("0\n");
  } else {
    for(int i = 1; i <= N; i++) {
    //edges[node].pop_back();
      edges[i].pop_back();
      res = count_cycles();
      if(res.first == 0) {
        final_nodes++;
        //res_nodes[node] = true;
        res_nodes[i] = true;
      }
      edges[i].push_back(half+i);
    }
    printf("%d\n", final_nodes);
    for(int i = 1; i <= N; i++)
      if(res_nodes[i]) printf("%d ", i);
  }
  return 0;
}