#define rep(i,n) for(int i=0;i<(int)(n);i++)
#define ALL(v) v.begin(),v.end()
typedef long long ll;
 
#include<bits/stdc++.h>
using namespace std;

int color[100010];
bool flag=true;

void dfs(int u,vector<vector<int>>& G){
  color[u]=1;
  for(auto x:G[u]){
    if(color[x]==1){
      flag=false;
      return;
    }
    if(color[x]==0) dfs(x,G);
  }
  color[u]=2;
}
 
int main(){
  ios::sync_with_stdio(false);
  std::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]--;
  }
  
  vector<vector<int>> G(n);
  rep(i,q) G[A[i]].push_back(B[i]);
  rep(i,n) if(color[i]==0) dfs(i,G);
  if(flag==true){
    cout<<-1<<endl;
    return 0;
  }
  
  int ok=q,ng=0;
  while(ok-ng>1){
    int mid=(ok+ng)/2;
    flag=true;
    rep(i,n) color[i]=0;
    vector<vector<int>> H(n);
    rep(i,mid) H[A[i]].push_back(B[i]);
    rep(i,n) if(color[i]==0) dfs(i,H);
    if(flag==true) ng=mid;
    else ok=mid;
  }
  cout<<ok<<endl;

  return 0;
}