#include <iostream>
#include <vector>

using namespace std;
vector<int> toposo(int n,vector<int> G[]){
    int i;
    vector<int> t,in(n);
    for(i=0;i<n;i++){
        for(int v:G[i]) in[v]++;
    }
    for(i=0;i<n;i++){
        if(!in[i]) t.push_back(i);
    }
    i = 0;
    while(i<t.size()){
        int u = t[i];
        for(int v:G[u]){
            in[v]--;
            if(in[v]==0) t.push_back(v);
        }
        i++;
    }
    return t;
}

vector<int> G[100010];
int a[100010],b[100010];
int main(){
    int i,n,q; cin >> n >> q;
    for(i=0;i<q;i++){
        cin >> a[i] >> b[i]; a[i]--; b[i]--;
        G[a[i]].push_back(b[i]);
    }
    vector<int> v = toposo(n,G);
    if(v.size()==n){
        cout << -1 << endl;
        return 0;
    }
    int le = 0,ri = q - 1;
    while(ri - le>1){
        int mid = (le + ri)/2;
        for(i=0;i<n;i++) G[i].clear();
        for(i=0;i<=mid;i++) G[a[i]].push_back(b[i]);
        vector<int> v = toposo(n,G);
        if(v.size()<n) ri = mid;
        else le = mid;
    }
    cout << ri + 1 << endl;
}