#include <bits/stdc++.h>
using namespace std;
int main(){
  int N, K;
  cin >> N >> K;
  vector<vector<int>> E(N);
  for (int i = 0; i < K; i++){
    int r, c;
    cin >> r >> c;
    r--;
    c--;
    E[r].push_back(c);
  }
  vector<int> d(N, 0);
  for (int i = 0; i < N; i++){
    for (int j : E[i]){
      d[j]++;
    }
  }
  vector<int> T;
  queue<int> Q;
  for (int i = 0; i < N; i++){
    if (d[i] == 0){
      Q.push(i);
    }
  }
  while (!Q.empty()){
    int v = Q.front();
    Q.pop();
    T.push_back(v);
    for (int w : E[v]){
      d[w]--;
      if (d[w] == 0){
        Q.push(w);
      }
    }
  }
  if (T.size() != N){
    cout << -1 << endl;
  } else {
    vector<int> dp(N, 1);
    for (int i = 0; i < N; i++){
      for (int j : E[T[i]]){
        dp[j] = max(dp[j], dp[T[i]] + 1);
      }
    }
    int ans = 0;
    for (int i = 0; i < N; i++){
      ans = max(ans, dp[i]);
    }
    cout << ans << endl;
  }
}