#include<cstdio>

#include<algorithm>

#define N (50010)

#define rg register

using namespace std;

int n,m,tot,top,color,ans,last[N],st[N],col[N],dfn[N],low[N],num[N];

int degree_out[N];

struct edge{

	int to,pre;

}e[N];

struct record{

	int u,v;

}r[N];

inline int read(){

	int k=0,f=1; char c=getchar();

	while(c<'0'||c>'9')c=='-'&&(f=-1),c=getchar();

	while('0'<=c&&c<='9')k=k*10+c-'0',c=getchar();

	return k*f;

}

inline int min(int x,int y){return x<y?x:y;}

inline void add(int x,int y){e[++tot]=(edge){y,last[x]}; last[x]=tot;}

void tarjan(int x){

	dfn[x]=low[x]=++tot; st[++top]=x;

	for(rg int i=last[x],to;i;i=e[i].pre)

		if(!dfn[to=e[i].to]) tarjan(to),low[x]=min(low[x],low[to]);

		else if(!col[to]) low[x]=min(low[x],dfn[to]);

	if(dfn[x]==low[x])

		for(color++;st[top+1]!=x;top--) col[st[top]]=color,num[color]++;

}

int main(){

	n=read(); m=read();

	for(rg int i=1,u,v;i<=m;i++) u=read(),v=read(),add(u,v),r[i].u=u,r[i].v=v;

	tot=0;

	for(rg int i=1;i<=n;i++) if(!col[i]) tarjan(i);

	for(rg int i=1,u,v;i<=m;i++){

		u=r[i].u; v=r[i].v;

		if(col[u]!=col[v]) degree_out[col[u]]++;

	}

	int ans=-1;

	for(rg int i=1;i<=color;i++) if(!degree_out[i]){

		printf("%d\n",num[i]);

		return 0;

	}

	puts("0");

	return 0;

}