#include <atcoder/scc>
#include <iostream>
using namespace std;

int A[100000];
int B[100000];

int main() {
    cin.tie(nullptr), ios::sync_with_stdio(false);
    int N, Q;
    cin >> N >> Q;
    for (int q = 0; q < Q; ++q) cin >> A[q] >> B[q];

    int ng = 0, ok = Q + 1;
    while (ok - ng > 1) {
        const int c = (ok + ng) / 2;
        atcoder::scc_graph g(N + 1);
        for (int e = 0; e < c; ++e) g.add_edge(A[e], B[e]);
        (int(g.scc().size()) < N + 1 ? ok : ng) = c;
    }
    cout << (ok == Q + 1 ? -1 : ok) << '\n';
}