//g++ 2.cpp -std=c++14 -O2 -I .
#include <bits/stdc++.h>
using namespace std;

#include <atcoder/all>
using namespace atcoder;

using ll = long long;
using ld = long double;

using vi = vector<int>;
using vvi = vector<vi>;
using vll = vector<ll>;
using vvll = vector<vll>;
using vld = vector<ld>;
using vvld = vector<vld>;
using vst = vector<string>;
using vvst = vector<vst>;

#define fi first
#define se second
#define pb push_back
#define eb emplace_back
#define pq_big(T) priority_queue<T,vector<T>,less<T>>
#define pq_small(T) priority_queue<T,vector<T>,greater<T>>
#define all(a) a.begin(),a.end()
#define rep(i,start,end) for(ll i=start;i<(ll)(end);i++)
#define per(i,start,end) for(ll i=start;i>=(ll)(end);i--)
#define uniq(a) sort(all(a));a.erase(unique(all(a)),a.end())

void dfs(vvi &g,vi &seen,int now,int par,int &flg){
  seen[now]=1;

  for(int next:g[now]){
    if(next==par){
      flg=0;
      return;
    }
    dfs(g,seen,next,now,flg);
  }
}

int main(){
  ios::sync_with_stdio(false);
  cin.tie(nullptr);

  int n,q;
  cin>>n>>q;

  vector<pair<int,int>> edge(q);
  rep(i,0,q){
    int a,b;
    cin>>a>>b;
    a--;
    b--;
    edge[i]={a,b};
  }

  int ng=0,ok=q+1;
  while(ok-ng>1){
    int check=(ok+ng)/2;
    vvi g(n);
    rep(i,0,check){
      auto [a,b]=edge[i];
      g[a].emplace_back(b);
    }

    cerr<<check<<endl;

    int flg=1;
    vi seen(n,0);
    rep(i,0,n){
      if(seen[i]==0){
        dfs(g,seen,i,-1,flg);
      }
    }

    if(flg==1){
      ng=check;
    }
    else{
      ok=check;
    }
  }

  if(ok==q+1){
    cout<<-1<<endl;
    return 0;
  }
  cout<<ok<<endl;
}