#include <cassert>
#include <cmath>
#include <algorithm>
#include <iostream>
#include <iomanip>
#include <climits>
#include <map>
#include <queue>
#include <set>
#include <cstring>
#include <vector>

using namespace std;
typedef long long ll;

int N, Q;
int A[100010];
int B[100010];
map<int, map<int, bool>> E;

void update_edges(int x) {
  for (int i = 0; i < Q; ++i) {
    int a = A[i];
    int b = B[i];

    if (i < x) {
      E[a][b] = true;
    } else {
      E[a][b] = false;
    }
  }
}

bool dfs(int v, vector<int> &visited) {
  visited[v] = 1;

  for (auto &[u, s] : E[v]) {
    if (not s) continue;

    if (visited[u] == 1) return true;
    if (visited[u] == 0 && dfs(u, visited)) return true;
  }

  visited[v] = 2;
  return false;
}

bool has_cycle() {
  vector<int> visited(N + 1, 0);

  for (int v = 1; v <= N; ++v) {
    if (visited[v] == 0 && dfs(v, visited)) return true;
  }

  return false;
}

int main() {
  cin >> N >> Q;

  for (int i = 0; i < Q; ++i) {
    cin >> A[i] >> B[i];
  }

  update_edges(Q);
  if (not has_cycle()) {
    cout << -1 << endl;
    return 0;
  }

  int ok = Q;
  int ng = 0;

  while (abs(ok - ng) >= 2) {
    int x = (ok + ng) / 2;
    update_edges(x);

    if (has_cycle()) {
      ok = x;
    } else {
      ng = x;
    }
  }

  cout << ok << endl;

  return 0;
}