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#include <algorithm>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <string>
#include <vector>
#include <queue>
using namespace std;

bool visited[1000000 + 2];
deque<int> D;
vector<int> G[1000000 + 2], H[1000000 + 2];
vector<int> comp[1000000 + 2];
int only_strong_comp;
bool in_strong_comp[1000000 + 2];
int postorder[1000000 + 2];
int postorder_id = 1;
vector<int> base_cycle;
bool push_cycle = false;
int first_cycle_item = 0;

void dfs1(int v) {
  if (visited[v]) {
    return;
  }

  visited[v] = true;

  for (size_t i = 0; i < G[v].size(); i++) {
    dfs1(G[v][i]);
  }

  D.push_front(v);
}

void dfs2(int v, int id) {
  if (visited[v]) {
    return;
  }

  visited[v] = true;
  comp[id].push_back(v);

  for (size_t i = 0; i < H[v].size(); i++) {
    dfs2(H[v][i], id);
  }
}

bool dfs3(int v) {
  visited[v] = true;

  for (size_t i = 0; i < G[v].size(); i++) {
    int w = G[v][i];

    if (visited[w] && postorder[w] == 0) {
      push_cycle = true;
      first_cycle_item = w;
      base_cycle.push_back(v);
      return true;
    }

    if (in_strong_comp[w] && !visited[w]) {
      if (dfs3(w)) {
        if (push_cycle) {
          base_cycle.push_back(v);
        }
        if (first_cycle_item == v) {
          push_cycle = false;
        }
        return true;        
      }
    }
  }

  postorder[v] = postorder_id++;
  return false;
}

bool dfs4(int v, int removed) {
  visited[v] = true;

  for (size_t i = 0; i < G[v].size(); i++) {
    int w = G[v][i];

    if (w == removed) {
      continue;
    }

    if (visited[w] && postorder[w] == 0) {
      return false;
    }
    
    if (in_strong_comp[w] && !visited[w]) {
      if (!dfs4(w, removed)) {
        return false;
      }
    }
  }

  postorder[v] = postorder_id++;
  return true;
}

int main() {
  int n, m;
  scanf("%d%d", &n, &m);

  for (int i = 0; i < m; i++) {
    int a, b;
    scanf("%d%d", &a, &b);
    G[a].push_back(b);
    H[b].push_back(a);
  }
  
  // Assign vertexes to strongly connected components.
  memset(visited, false, (n + 2) * sizeof(bool));
  for (int i = 1; i <= n; i++) {
    dfs1(i);
  }

  memset(visited, false, (n + 2) * sizeof(bool));
  int id = 0;
  for (int v : D) {
    if (!visited[v]) {
      dfs2(v, id);
      id++;
    }
  }

  // Check the number of components and only proceed, if there is exactly one larger than a single vertex.
  int max_comp_id = -1;
  int max_comp_size = -1;
  for (int i = 0; i < id; i++) {
    if (comp[i].size() > 1) {
      if (max_comp_size == -1) {
        max_comp_id = i;
        max_comp_size = comp[i].size();
      } else {
        printf("0\n");
        return 0;
      }
    }
  }

  if (max_comp_size == -1) {
    printf("NIE\n");
    return 0;
  }
  only_strong_comp = max_comp_id;
 
  memset(in_strong_comp, false, (n + 2) * sizeof(bool));
  for (size_t i = 0; i < comp[only_strong_comp].size(); i++) {
    in_strong_comp[comp[only_strong_comp][i]] = true;
    //printf("In strong component: %d\n", comp[only_strong_comp][i]);
  }

  // Find the base cycle in the component.
  memset(visited, false, (n + 2) * sizeof(bool));
  memset(postorder, 0, (n + 2) * sizeof(int));
  postorder_id = 1;
  dfs3(comp[only_strong_comp][0]);

  /*printf("@@@ base cycle size: %d\n", base_cycle.size());
  for (size_t i = 0; i < base_cycle.size(); i++) {
    printf("@@@ %d\n", base_cycle[i]);
  }*/

  // For each component of the cycle, check if it is part of the solution.
  vector<int> wyn;
  for (size_t i = 0; i < base_cycle.size(); i++) {
    memset(visited, false, (n + 2) * sizeof(bool));
    memset(postorder, 0, (n + 2) * sizeof(int));
    postorder_id = 1;

    bool success = true;

    for (size_t j = 0; j < comp[only_strong_comp].size(); j++) {
      if (base_cycle[i] == comp[only_strong_comp][j]) {
        continue;
      }

      if (visited[comp[only_strong_comp][j]]) {
        continue;
      }

      if (!dfs4(comp[only_strong_comp][j], base_cycle[i])) {
        success = false;
        break;
      }
    }

    if (success) {
      wyn.push_back(base_cycle[i]);
    }
  }
  
  sort(wyn.begin(), wyn.end());
  printf("%d\n", wyn.size());
  for (size_t i = 0; i < wyn.size(); i++) {
    printf("%d ", wyn[i]);
  }

  return 0;
}