#include <iostream>
#include <vector>
#include <cmath>
#include <map>
#include <set>
#include <iomanip>
#include <queue>
#include <algorithm>
#include <numeric>
#include <deque>
#include <sstream>

using namespace std;

template<typename T> using pq = priority_queue<T, vector<T>, greater<T>>;

vector<int> topological_sort(vector<vector<int>> &E){
    int N = E.size(), C;
    vector<int> deg(N), res;
    pq<int> que;

    for (int i=0; i<N; i++){
        for (auto j : E[i]) deg[j]++;
    }


    for (int i=0; i<N; i++){
        if (deg[i] == 0) que.push(i);
    }

    while(!que.empty()){
        C = que.top();
        que.pop();
        res.push_back(C);
        for (auto x : E[C]){
            deg[x]--;
            if (deg[x] == 0) que.push(x);
        }   
    }

    return res;
}

int N;
vector<int> A, B;

bool solve(int c){

    vector<vector<int>> E(N);
    vector<int> ans;

    for (int i=0; i<c; i++){
        E[A[i]].push_back(B[i]);
    }

    ans = topological_sort(E);

    return (ans.size() != N);
}

int main(){

    int Q, l, r, c;
    cin >> N >> Q;
    A.resize(Q); B.resize(Q);
    for (int i=0; i<Q; i++){
        cin >> A[i] >> B[i];
        A[i]--; B[i]--;
    }

    if (!solve(Q)){
        cout << -1 << endl;
        return 0;
    }

    r = Q; l = 0;
    while(r-l>1){
        c = (r+l)/2;
        if (solve(c)) r=c;
        else l=c;
    }
    cout << r << endl;

    return 0;
}