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

int main() {
  int n, k;
  cin >> n >> k;
  vector<vector<int>> E(n);
  vector<int> in(n, 0);
  for (int i = 0; i < k; i++) {
    int a, b;
    cin >> a >> b;
    a--, b--;
    E[a].push_back(b);
    in[b]++;
  }
  queue<int> que;
  vector<int> d(n, -1);
  for (int i = 0; i < n; i++) {
    if (in[i] == 0) {
      que.push(i);
      d[i] = 0;
    }
  }
  while (not que.empty()) {
    int u = que.front();
    que.pop();
    for (auto v : E[u]) {
      if (d[v] < d[u] + 1) d[v] = d[u] + 1;
      if (--in[v] == 0) {
        que.push(v);
      }
    }
  }
  int ans = 0;
  for (int i = 0; i < n; i++) {
    if (in[i] > 0) {
      ans = -1;
      break;
    }
    if (ans < d[i] + 1) ans = d[i] + 1;
  }
  cout << ans << "\n";
}