#include <bits/stdc++.h>
#include <atcoder/scc>
using namespace std;
using ll = long long;
#define rep(i,n) for (int i = 0; i < n; i++)
#define rrep(i,n) for (int i = n-1; i >= 0; i--)
#define rep2(i,a,b) for (int i = a; i < b; i++)
#define rrep2(i,a,b) for (int i = a-1; i >= b; i--)
#define rep3(i,a,b,c) for (int i = a; i < b; i+=c)
#define rrep3(i,a,b,c) for (int i = a-1; i >= b; i-=c)
#define all(v) v.begin(),v.end()
#define rall(v) v.rbegin(),v.rend()
template<class T> bool chmax(T &a, T b){if (a < b){a = b;return true;} else return false;}
template<class T> bool chmin(T &a, T b){if (a > b){a = b;return true;} else return false;}
template<class T> T pow(T x, ll y){if (y == 0) return 1;T p = pow(x,y/2);if (y%2 == 0) return p*p;else return p*p*x;}

int main(){
    ios::sync_with_stdio(false);
    cin.tie(nullptr);
    int n,q; cin >> n >> q;
    vector<int> A(q), B(q);
    rep(i,q){
        cin >> A[i] >> B[i];
        A[i]--;
        B[i]--;
    }
    int ok = 1, ng = q+1;
    while (ng-ok > 1){
        int mid = (ng+ok)/2;
        atcoder::scc_graph g(n);
        rep(i,mid) g.add_edge(A[i],B[i]);
        if (g.scc().size() != n) ng = mid;
        else ok = mid;
    }
    if (ok != q) cout << ok+1 << endl;
    else cout << -1 << endl;
}