#include <stdio.h>
#include <bits/stdc++.h>
#include <atcoder/all>
using namespace atcoder;
using mint = modint998244353;
using namespace std;
#define rep(i,n) for(int i=0;i<(n);i++)
#define Inf 1000000000000000


int main(){
	
	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 = q+1,ng = 0;
	while(ok-ng>1){
		int mid = (ok+ng)/2;
		scc_graph S(n);
		rep(i,mid){
			S.add_edge(a[i],b[i]);
		}
		if(S.scc().size()!=n)ok = mid;
		else ng= mid;
	}
	
	if(ok==q+1)ok = -1;
	cout<<ok<<endl;
	
	return 0;
}