#include<bits/stdc++.h>
using namespace std;
const int N=1e4+5,M=5e4+5;
struct edge{
	int x,y,pre;
}a[M];
int last[N],alen;
void ins(int x,int y){
	a[++alen]=edge{x,y,last[x]};
	last[x]=alen;
}
int dfn[N],low[N],v[N],id;
int sta[N],top;
int cnt,c[N],siz[N],cd[N];
void tarjan(int x){
	dfn[x]=low[x]=++id,v[x]=1;
	sta[++top]=x;
	for(int k=last[x];k;k=a[k].pre){
		int y=a[k].y;
		if(!dfn[y]){
			tarjan(y);
			low[x]=min(low[x],low[y]);
		}
		else if(v[y]){
			low[x]=min(low[x],dfn[y]);
		}
	}
	if(dfn[x]==low[x]){
		cnt++;
		int y;
		do{
			y=sta[top--];
			v[y]=0;
			c[y]=cnt;
			siz[cnt]++;
		}while(y!=x);
	}
}
int main(){
	int n,m;scanf("%d%d",&n,&m);
	alen=1;memset(last,0,sizeof(last));
	for(int i=1;i<=m;i++){
		int x,y;scanf("%d%d",&x,&y);
		ins(x,y);
	}
	for(int i=1;i<=n;i++){
		if(!dfn[i])tarjan(i);
	}
	for(int x=1;x<=n;x++){
		for(int k=last[x];k;k=a[k].pre){
			int y=a[k].y;
			if(c[x]==c[y])continue;
			cd[c[x]]++;
		}
	}
	int tot=0,pos;
	for(int i=1;i<=cnt;i++){
		if(!cd[i]){
			tot++;
			pos=i;
		}
	}
	if(tot==1)printf("%d",siz[pos]);
	else printf("0");
	return 0;
}