#include <bits/stdc++.h>
using namespace std;
bool dfs(int i, vector<int>& seen, vector<vector<int> >& g)
{
    seen[i] = 1;
    for (auto& j : g[i]) {
        if (seen[j] == 1 || (seen[j] == 0 && dfs(j, seen, g))) {
            return true;
        }
    }
    seen[i] = 2;
    return false;
}
int main()
{
    int n, q;
    cin >> n >> q;
    vector<pair<int, int> > edges(q);
    for (int i = 0; i < q; i++) {
        int a, b;
        cin >> a >> b;
        edges[i] = { a - 1, b - 1 };
    }
    int sq = sqrtl(q);
    int block_size = (q + sq - 1) / sq;
    auto check = [&](int i) {
        vector<vector<int> > g(n);
        for (int j = 0; j < i; j++) {
            g[edges[j].first].push_back(edges[j].second);
        }
        vector<int> seen(n, 0);
        bool has_cycle = false;
        for (int i = 0; i < n; i++) {
            if (seen[i] == 0 && dfs(i, seen, g)) {
                has_cycle = true;
                break;
            }
        }
        return has_cycle;
    };
    int idx = 0;
    for (int i = 0; i <= sq; i++) {
        if (check(min(q, i * block_size))) {
            idx = (i - 1) * block_size;
            break;
        }
    }
    for (int i = 0; i <= block_size; i++) {
        if (check(min(q, idx + i))) {
            cout << idx + i << endl;
            return 0;
        }
    }
    cout << -1 << endl;
}