#include <bits/stdc++.h>
using namespace std;
int main(){
  int N, Q;
  cin >> N >> Q;
  vector<int> A(Q), B(Q);
  for (int i = 0; i < Q; i++){
    cin >> A[i] >> B[i];
    A[i]--;
    B[i]--;
  }
  int tv = Q + 1, fv = 0;
  while (tv - fv > 1){
    int mid = (tv + fv) / 2;
    vector<vector<int>> E(N);
    vector<int> d(N, 0);
    for (int i = 0; i < mid; i++){
      E[A[i]].push_back(B[i]);
      d[B[i]]++;
    }
    queue<int> q;
    for (int i = 0; i < N; i++){
      if (d[i] == 0){
        q.push(i);
      }
    }
    int cnt = 0;
    while (!q.empty()){
      int v = q.front();
      q.pop();
      cnt++;
      for (int w : E[v]){
        d[w]--;
        if (d[w] == 0){
          q.push(w);
        }
      }
    }
    if (cnt < N){
      tv = mid;
    } else {
      fv = mid;
    }
  }
  if (tv == Q + 1){
    cout << -1 << endl;
  } else {
    cout << tv << endl;
  }
}