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

class GraphCycle {
  private:
    const vector<vector<int>>& G;
    std::vector<bool> visited, seen;
    bool result;
    bool dfs(int v) {
        if (seen[v]) return true;
        if (visited[v]) return false;
        visited[v] = seen[v] = true;
        for (const auto& e : G[v]) {
            if (dfs(e)) return true;
        }
        seen[v] = false;
        return false;
    }
    void init() {
        const int N = G.size();
        visited.assign(N, false);
        seen.assign(N, false);
        result = false;
        for (int i = 0; i < N; ++i) {
            if (dfs(i)) {
                result = true;
                break;
            }
        }
    }
  public:
    GraphCycle(const vector<vector<int>>& G) : G(G) { init(); }
    bool has_cycle() const { return result; }
};

int main() {
    int N, Q; cin >> N >> Q;
    vector<pair<int, int>> A(Q);
    for (int i = 0; i < Q; ++i) {
        cin >> A[i].first >> A[i].second;
        --A[i].first; --A[i].second;
    }
    int ok = Q + 1, ng = 0;
    while (ok - ng > 1) {
        int mid = (ok + ng) >> 1;
        vector<vector<int>> G(N);
        for (int i = 0; i < mid; ++i) G[A[i].first].push_back(A[i].second);
        GraphCycle cyc(G);
        if (cyc.has_cycle()) ok = mid;
        else ng = mid;
    }
    if (ok == Q + 1) puts("-1");
    else cout << ok << endl;
}