#include <bits/stdc++.h>
#define ll long long
using namespace std;
const int MAXN=1e4+5;
const int INF=2147483647; 
int sum,low[MAXN],n,m,dfn[MAXN],cnt,grade[MAXN],ans,zx=INF,Sum;
bool vis[MAXN];
vector<int>from[MAXN],to[MAXN];
int findfa(int u){
	int ans=0;
	if(vis[u]) return 0;
	vis[u]=true,Sum++;
	for(int i=0;i<to[u].size();i++){
		if(low[u]!=low[to[u][i]]) ans++;
		else ans+=findfa(to[u][i]);
	}
	return ans;
}
void bfs(int u,int k){
	dfn[u]=low[u]=++cnt,grade[u]=k;
	for(int i=0;i<to[u].size();i++){
		if(!dfn[to[u][i]]){
			bfs(to[u][i],k);
			low[u]=min(low[u],low[to[u][i]]);
		}
		if(grade[to[u][i]]==k) low[u]=min(low[u],low[to[u][i]]);
	}
	if(low[u]==dfn[u]){
		Sum=0;
		if(!findfa(u)) sum+=Sum;
	}
}
int main(){
	cin>>n>>m;
	for(int x,y,i=1;i<=m;i++){
		scanf("%d%d",&x,&y);
		to[x].push_back(y);
	}
	int du=0;
	for(int i=1;i<=n;i++) if(!to[i].size()) du++;
	if(du>=2){
		cout<<"0";
		return 0;
	}
	for(int du=0,ans=0,i=1;i<=n;i++)
		if(!dfn[i]){
			if(du==2){
				cout<<0;
				return 0; 
			}
			du++;
			bfs(i,++ans);
		}
	cout<<sum;
	return 0;
}
/*
6 7
1 2
1 3
3 4
4 1
5 4
2 6 
6 1
*/