#include <bits/stdc++.h>
using namespace std;
const int N = 2e5;
bool vis[N];
int n, m, ans;
vector<int> top;
vector<pair<int, int> > adj[N];
void dfs(int u) {
  vis[u] = 1;
  for (int i = 0; i < adj[u].size(); i++) {
    int v = adj[u][i].first;
    if (!vis[v]) dfs(v);
  }
  top.push_back(u);
}
int main() {
  cin >> n >> m;
  for (int i = 1; i <= m; i++) {
    int u, v;
    cin >> u >> v;
    adj[u].push_back({v, i});
  }
  for (int i = 1; i <= n; i++)
    if (!vis[i]) dfs(i);
  for (int i = n - 1; i > 0; i--) {
    int u = top[i], v = top[i - 1], j = 0;
    for (j = 0; j < adj[u].size(); j++) {
      if (adj[u][j].first == v) {
        ans = max(adj[u][j].second, ans);
        break;
      }
    }
    if (j == adj[u].size()) {
      cout << "-1\n";
      return 0;
    }
  }
  cout << ans << endl;
  return 0;
}
