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

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>used(N);
        bool f = false;
        for(int i = 0; i < N; i++) {
            if(!used[i]) {
                queue<int>que;
                que.push(i);
                used[i] = true;
                while (!que.empty()) {
                    int x = que.front();
                    que.pop();
                    for(int j:road[x]) {
                        if(used[j]) {
                            f = true;
                            break;
                        }
                        used[j] = true;
                        que.push(j);
                    }
                    if(f) {
                        break;
                    }
                }
                if(f) {
                    break;
                }
            }
        }
        if(f) {
            r = mid;
        }
        else {
            l = mid;
        }
    }
    cout << ((r == Q)?-1:r+1) << endl;
}