#include<iostream>
#include<vector>
#include<algorithm>
#include<stdio.h>
using namespace std;
bool roditel[300005];
vector<int>susedi[300005];
int cmp(int a, int b)
{
    return susedi[a].size()<susedi[b].size();
}
int blq[300005];
int main(){
    cin.tie(0);
    ios::sync_with_stdio(false);
    freopen("company.in",  "r", stdin);
  freopen("company.out", "w", stdout);
int n,m;
cin>>n>>m;
for(int i=0;i<m;i++)
{
    int a,b;
    cin>>a>>b;
    susedi[b].push_back(a);
}
for(int i=2;i<=n;i++)
{
    if(susedi[i].size()==0)
    {
        cout<<-1<<endl;
        return 0;
    }
}
for(int i=0;i<n;i++)
    blq[i]=i+1;
sort(blq, blq+n, cmp);
int otg=0;
for(int i=0;i<n;i++)
{
    for(int j=0;j<susedi[blq[i]].size();j++)
    {
        if(!roditel[susedi[blq[i]][j]])
        {
            roditel[susedi[blq[i]][j]]=true;
            otg++;
            break;
        }
    }
}
cout<<n-otg<<endl;
return 0;
}