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#include <bits/stdc++.h>
using namespace std;

const int MAXN = 500 * 1000;

vector<int> graph[MAXN], rev_graph[MAXN];
int color[MAXN];
vector<int> ordering;
int scc[MAXN], scc_size[MAXN];
int only_scc;
int in[MAXN];
int dfs_result[MAXN];
//int has_back[MAXN];
int has_back2[MAXN];
int dfs_time;
int left_branch;
int min_back, max_back;
vector<pair<int, int>> loops;

bool answer_is_0;

void rev_dfs(int u) {
  if (color[u])
    return;
  color[u] = 1;
  for (int v : rev_graph[u])
    rev_dfs(v);
  ordering.push_back(u);
}

void dfs(int u, int scc_num) {
  if (color[u])
    return;
  color[u] = 1;
  scc[u] = scc_num;
  for (int v : graph[u])
    dfs(v, scc_num);
}

void dfs2(int u) {
  color[u] = 1;
  in[u] = dfs_time++;
  dfs_result[u] = 0;
  //has_back[u] = true;
  has_back2[u] = MAXN;
  for (int v : graph[u]) {
    if (scc[v] != only_scc)
      continue;
    if (!color[v]) {
      dfs2(v);
      if (in[v] >= left_branch) {
        dfs_result[u] = max(dfs_result[u], dfs_result[v]);
      //  has_back[u] = min(has_back[u], has_back[v]);
        has_back2[u] = min(has_back2[u], has_back2[v]);
      }
    } else {
      if (color[v] == 1)  // powrotna
        if (in[v] < left_branch || left_branch == -1) {
          has_back2[u] = min(has_back2[u], in[v]);
    //      has_back[u] = false;
          max_back = max(max_back, in[v]);
        } else {
          answer_is_0 = true;
        }
      if (color[v] == 2) {
        if (in[v] < left_branch)
          dfs_result[u] = max(dfs_result[u], in[v]);
        else {
          dfs_result[u] = max(dfs_result[u], dfs_result[v]);
          has_back2[u] = min(has_back2[u], has_back2[v]);
        }
      }
    }
  }
  if (has_back2[u] <= in[u])
    min_back = min(min_back, in[u]);
  // if (has_back[u] == 0)
  //   min_back = min(min_back, in[u]);
  if (left_branch == -1)
    left_branch = dfs_time;
  color[u] = 2;
  if (in[u] < left_branch && dfs_result[u] > in[u])
    loops.emplace_back(in[u], dfs_result[u]);
}

int main() {
  ios_base::sync_with_stdio(false);
  int n, m;
  cin >> n >> m;
  for (int i = 0; i < m; ++i) {
    int a, b;
    cin >> a >> b;
    --a;
    --b;
    graph[a].push_back(b);
    rev_graph[b].push_back(a);
  }
 
  fill_n(color, n, 0);
  for (int i = 0; i < n; ++i)
    rev_dfs(i);
  reverse(ordering.begin(), ordering.end());
  fill_n(color, n, 0);
  for (int u : ordering)
    dfs(u, u);
 
  //  for (int i =0 ;i<n;++i) cerr << i << " " << scc[i] << endl;

  fill_n(scc_size, n, 0);
  for (int i = 0; i < n; ++i)
    scc_size[scc[i]]++;
  int count2 = 0;
  for (int i = 0; i < n; ++i)
    if (scc_size[i] >= 2)
      ++count2;
  if (count2 == 0) {
    cout << "NIE" << endl;
    return 0;
  }
  if (count2 >= 2) {
    cout << 0 << endl;
    cout << endl;
    return 0;
  }
  
  for (int i = 0; i < n; ++i)
    if (scc_size[i] >= 2)
      only_scc = i;

  dfs_time = 0;
  left_branch = -1;
  min_back = n - 1;
  max_back = 0;
  fill_n(color, n, 0);
  answer_is_0 = false;
  dfs2(only_scc);
 /* 
  for (int i = 0; i < n; ++i)
    if (scc[i] == only_scc && i < 100)
      cerr << "in[" << i <<"] = " << in[i]<<endl;
  cerr <<"lb = " << left_branch << endl;
  cerr << "min_back = " << min_back << endl;
  cerr << "max_back = " << max_back << endl;
  cerr << "loops:" << endl;
  for (auto i : loops) cerr << i.first << " " <<i.second << endl;
  //*/

  if (answer_is_0) {
    cout << 0 << endl;
    cout << endl;
    return 0;
  }

  vector<int> counting(left_branch);
  for (pair<int, int> loop : loops) {
    counting[loop.first + 1]++;
    counting[loop.second]--; 
  }
  if (min_back < left_branch - 1)
    counting[min_back + 1]++;
  counting[0]++;
  counting[max_back]--;
  int sum = 0;
  vector<bool> ok(left_branch);
  for (int i = 0; i < left_branch; ++i) {
    sum += counting[i];
    if (sum == 0)
      ok[i] = true;
    else
      ok[i] = false;
  }
  vector<int> solution;
  for (int i = 0; i < n; ++i)
    if (scc[i] == only_scc && in[i] < left_branch && ok[in[i]])
      solution.push_back(i);
  cout << solution.size() << endl;
  for (int i = 0; i < solution.size(); ++i) {
    if (i > 0) cout << " ";
    cout << 1 + solution[i];
  }
  cout << endl;
}