#include <bits/stdc++.h>
using namespace std;

bool dfs(int n,vector<vector<int>>&road,vector<bool>&use1,vector<bool>&use2) {
    if(use2[n]) return true;
    use1[n] = use2[n] = true;
    bool f = false;
    for(int i:road[n]) {
        if(use2[i] || (!use1[n] && dfs(i,road,use1,use2))) {
            f = true;
            break;
        }
    }
    use2[n] = false;
    return f;
}

int main() {
    ios::sync_with_stdio(false);
    cin.tie(nullptr);
    int N,Q;
    cin >> N >> Q;
    vector<int>A(Q),B(Q);
    for(int i = 0; i < Q; i++) {
        cin >> A[i] >> B[i];
        A[i]--;
        B[i]--;
    }
    int l = -1,r = Q;
    while (l+1 < r) {
        int mid = (l+r)/2;
        vector<vector<int>>road(N);
        for(int i = 0; i <= mid; i++) {
            road[A[i]].push_back(B[i]);
        }
        vector<bool>use1(N),use2(N);
        bool f = false;
        for(int i = 0; i < N; i++) {
            if(!use1[i] && dfs(i,road,use1,use2)) {
                f = true;
                break;
            }
        }
        if(f) {
            r = mid;
        }
        else {
            l = mid;
        }
    }
    cout << ((r == Q)?-1:r+1) << endl;
}