#include<iostream>
#include<queue>
using namespace std;
const int N=10005,M=100005;
int End[M],Next[M],Head[M],idx;
int Done[N],low[N],Times[N],id[N],Size[N],timestamp,scc;
int Indu[M],Outdu[M];
int n,m;
deque<int> dq;
inline void add(int a,int b){
	idx++;End[idx]=b;Next[idx]=Head[a];Head[a]=idx;
}
inline void tarjan(int u){
	Times[u]=low[u]=++timestamp;
	dq.push_back(u);Done[u]=1;
	for(int i=Head[u];i;i=Next[i]){
		int j=End[i];
		if(!Times[j]){
			tarjan(j);
			low[u]=min(low[u],low[j]);
		}else if(Done[j]==1)low[u]=min(low[u],Times[j]);
	}
	if(Times[u]==low[u]){
		++scc;int y;
		do{
			y=dq.back();dq.pop_back();
			Done[y]==0;id[y]=scc;
			Size[scc]++;
		}while(y!=u);
	}
}
int main(){
	std::ios::sync_with_stdio(false);
	std::cin.tie(0);std::cout.tie(0);
	cin>>n>>m;
	for(int i=1;i<=m;i++){
		int a,b;cin>>a>>b;add(a,b);
	}
	for(int i=1;i<=n;i++){
		if(!Times[i]) tarjan(i);
	}
	for(int x=1;x<=n;x++){
		for(int i=Head[x];i;i=Next[i]){
			int y=End[i];
			if(id[x]==id[y])continue;
			add(id[x],id[y]);Outdu[id[x]]++;
		}
	}int cnt=0,ins;
	for(int i=1;i<=scc;i++){
		if(Outdu[i]==0) cnt++,ins=i;
	}
	cout<<(cnt==1?Size[ins]:0)<<"\n";
	return 0;
}