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

#ifdef _RUTHEN
#include "debug.hpp"
#else
#define show(...) true
#endif

vector<int> topological_sort(vector<vector<int>> &G) {
    int N = (int)G.size();
    vector<int> deg(N, 0);
    // 入次数をメモ
    for (int i = 0; i < N; i++) {
        for (int &v : G[i]) deg[v]++;
    }
    // 入次数0の点の集合を作成
    queue<int> que;
    for (int i = 0; i < N; i++) {
        if (deg[i] == 0) que.push(i);
    }
    vector<int> res;
    while (!que.empty()) {
        int v = que.front();
        que.pop();
        res.push_back(v);
        for (int &nx : G[v]) {
            // 入次数を減らす
            deg[nx]--;
            // 新たな入次数0の点を追加
            if (deg[nx] == 0) que.push(nx);
        }
    }
    return res;
}

using ll = long long;
#define rep(i, n) for (int i = 0; i < (n); i++)
template <class T> using V = vector<T>;

int main() {
    ios::sync_with_stdio(false);
    cin.tie(0);
    int N, Q;
    cin >> N >> Q;
    V<int> a(Q), b(Q);
    rep(i, Q) {
        cin >> a[i] >> b[i];
        a[i]--, b[i]--;
    }
    int ok = Q + 1, ng = 0;
    while (ok - ng > 1) {
        int md = (ok + ng) / 2;
        V<V<int>> G(N);
        rep(i, md) G[a[i]].push_back(b[i]);
        V<int> res = topological_sort(G);
        if (res.size() == N) {
            ng = md;
        } else {
            ok = md;
        }
    }
    cout << (ok == Q + 1 ? -1 : ok) << '\n';
    return 0;
}