#include <bits/stdc++.h>
using namespace std;
long long N = 1e5 + 10;
vector<vector<long long> > v1(N, vector<long long>(2));
vector<vector<long long> > v;
long long dfs(long long s, long long n, vector<long long>& level) {
  long long val = 0;
  for (auto u : v[s]) {
    if (level[u] == 0) {
      val = max(val, dfs(u, n, level));
    } else if (level[u]) {
      val = max(val, level[u]);
    }
  }
  level[s] = val + 1;
  return level[s];
}
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);
  long long n, m;
  cin >> n >> m;
  for (long long i = 1; i <= m; i++) {
    cin >> v1[i][0] >> v1[i][1];
  }
  long long low = 1, high = m, ans = -1;
  while (low <= high) {
    long long mid = (low + high) / 2;
    v.clear();
    v.resize(n + 1);
    vector<long long> par(n + 1, -1), level(n + 1);
    for (long long i = 1; i <= mid; i++) {
      v[v1[i][0]].push_back(v1[i][1]);
      par[v1[i][1]] = v1[i][0];
    }
    for (long long i = 1; i <= n; i++) {
      if (par[i] == -1) {
        if (dfs(i, n, level) == n) {
          ans = mid;
          high = mid - 1;
        } else {
          low = mid + 1;
        }
        break;
      }
    }
  }
  cout << ans;
  return 0;
}
