#include<bits/stdc++.h>
using namespace std;
#include <atcoder/all>
using namespace atcoder;
using mint=modint998244353; //1000000007;
using ll=long long;
using pp=pair<int,int>;
#define sr string 
#define vc vector
#define fi first
#define se second
#define rep(i,n) for(int i=0;i<(int)n;i++)
#define pb push_back
#define all(v) v.begin(),v.end()
#define pque priority_queue
#define bpc(a) __builtin_popcount(a)
int main(){
  int n,q;cin>>n>>q;
  vc v(n,vc<pp>(0));
  rep(i,q){
    int a,b;cin>>a>>b; a--; b--;
    v[a].pb({b,i}); 
  }
  int ac=q+1,wa=0;
  while(ac-wa>1){
    int x=(ac+wa)/2;
    vc<int>c(n,0); bool ok=false;
    auto f=[&](auto f,int a)->void{
      c[a]=1;
      for(auto [au,i]:v[a])if(i<x){
        if(c[au]==1)ok=true;
        if(c[au]==0)f(f,au);
      }
      c[a]=-1;
    };
    rep(i,n)if(c[i]==0)f(f,i);
    if(ok)ac=x;
    else wa=x;
  }
  if(ac==q+1)cout<<-1;
  else cout<<ac;
}