某站

#include<iostream>
#include<cstdio>
#include<algorithm>
#include<cmath>
#include<cstring>
#include<cstdlib>
#include<ctime>
#include<iomanip>
using namespace std;
int cnt,a[105],ml[4505],mr[4505];

int main(){
    int n,m;
    cin>>n>>m;
    for(int i=1;i<=m;i++){
        cin>>ml[i]>>mr[i];
        a[mr[i]]++;
    }
    for(int i=1;i<=n;i++){
        for(int j=1;j<=m;j++){
            if(mr[j]==i){
                a[i]=max(a[i],a[ml[j]]+1);
            }
        }
    }
    sort(a+1,a+1+n);
    for(int i=1;i<=n;i++){
        if(a[i]==i-1 && a[i+1]!=i-1)cnt++;
    }
    cout<<cnt;
    return 0;
}