#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; }