#include<cstdio>
#include<algorithm>
#include<cstring>
#include<vector>
#define N 10005
using namespace std;
int n,m,a,b,head[N],to[N],nxt[N],tot;
int rh[N],rt[N],rn[N],cmp[N],f,mmp,ans;
bool vis[N]; 
vector<int > out;
void add(int u,int v){
	to[++tot]=v;rt[tot]=u;
	nxt[tot]=head[u];rn[tot]=rh[v];
	head[u]=tot;rh[v]=tot;
}
void dfs(int x){
	vis[x]=1;
	for(int i=head[x];i;i=nxt[i]){
		if(!vis[to[i]])dfs(to[i]);
	}
	out.push_back(x);
}
void rd(int x,int f){
	vis[x]=1;cmp[x]=f;
	for(int i=rh[x];i;i=rn[i]){
		if(!vis[x])rd(rt[i],f);
	}
}
void kosaraju(){
	out.clear();
	memset(vis,0,sizeof(vis));
	for(int i=1;i<=n;i++){
		if(!vis[i])dfs(i);
	}
	memset(vis,0,sizeof(vis));
	for(int i=out.size()-1;i>=0;i--){
		if(!vis[out[i]])rd(i,f++);
	}
	f--;
}
void dfs2(int x){
	vis[x]=1;
	for(int i=rh[x];i;i=rn[i]){
		if(!vis[x])dfs2(rt[i]);
	}
}
signed main(){
	scanf("%d%d",&n,&m);
	for(int i=1;i<=m;i++){
		scanf("%d%d",&a,&b);
		add(a,b);
	}
	kosaraju();
	for(int i=1;i<=n;i++)if(cmp[i]==f)mmp=i,ans++;
	memset(vis,0,sizeof(vis));
	dfs2(mmp);
	for(int i=1;i<=n;i++){
		if(!vis[i]){
			printf("0");return 0;
		}
	}
	printf("%d",ans);
	return 0;
}