#include <atcoder/all>
#include <bits/stdc++.h>
#define rep(i, a, b) for (ll i = (ll)(a); i < (ll)(b); i++)
using namespace atcoder;
using namespace std;

typedef long long ll;

int main() {
    ios::sync_with_stdio(false);
    cin.tie(nullptr);
    int n, m;
    cin >> n >> m;
    vector<int> u(m), v(m);
    rep(i, 0, m) cin >> u[i] >> v[i], u[i]--, v[i]--;
    int ac = 0, wa = m + 1;
    while (wa - ac > 1) {
        scc_graph gr(n);
        int mid = (ac + wa) / 2;
        rep(i, 0, mid) gr.add_edge(u[i], v[i]);
        auto scc = gr.scc();
        if ((int)scc.size() == n) {
            ac = mid;
        } else {
            wa = mid;
        }
    }
    int ans = (wa == m + 1) ? -1 : wa;
    cout << ans << endl;
}