#include<atcoder/scc>
#include<bits/stdc++.h>
using namespace std;
int main()
{
    int N, Q;
    cin >> N >> Q;
    vector<pair<int, int>> V(Q);
    for(auto&[a, b]: V) cin >> a >> b;
    int ok = Q+1, ng = 0;
    auto can = [&](int x)
    {
        atcoder::scc_graph G(N);
        for(int i=0; i<x; ++i)
        {
            auto [a, b] = V[i];
            G.add_edge(a-1, b-1);
        }
        return (int)G.scc().size() != N;
    };
    while(ng+1!=ok)
    {
        int mi = (ok+ng)/2;
        if(can(mi)) ok = mi;
        else ng = mi;
    }
    if(ok == Q+1) ok = -1;
    cout << ok << endl;
}