#include <stdio.h>
#define NL   0
#define MN   10002 

int stack [MN],top= 0 ;
int pre[MN],low[MN],cnt0 = 1;
int sc[MN],cnt1 = 1;
int n,m ;

typedef struct Edge * Link ;
struct Edge{
	int node ;
	Link next ;
	Edge( int n = 0, Link p = NULL ):node(n),next(p){}
} head[MN]  ; 
int count[MN],dcount[MN],ccount[MN] ;

void InsertEdge( Edge h[],int x,int y){
	h[y].next = new Edge(x,h[y].next) ;
}

void SCR( int x ){      //求强连通分量
	low[x] = pre[x] = cnt0 ++ ; stack[top++] = x ;
	int minn = low[x] ; int nx,v ;
	for( Link p = head[x].next ; p ; p = p->next ){
		nx = p->node ; 
		if( pre[nx] == NL )SCR(nx) ;
		if( low[nx] == n+1 && ccount[nx] != -1 ){
			ccount[x]+=dcount[sc[nx]] ;
			ccount[nx] = -1 ;
		} 
		if( low[nx] < minn ) minn = low[nx];
	}
	if( minn < low[x] ){
		low[x] = minn ; return ;
	}
	while( stack[top] != x ){
		sc[ v = stack[--top] ] = cnt1 ; low[v] = n+1 ; //置为最大
		++count[cnt1] ; if( v!= x)ccount[x] += ccount[v] ; 
	}
	dcount[cnt1++] = ccount[x] ;
}
void Initial(){
	int i ,j;
	for( i = 1; i <= n ; i++ )
		ccount[i] = 1 ;
	for( i = 1; i <= n ;i ++ )
		if( sc[i] == 0 )SCR(i) ;
}

int main(){
	freopen("in.txt","r",stdin) ;
	scanf("%d%d",&n,&m ) ;  int x,y ;
	while (m--){
		scanf("%d%d",&x,&y) ;
		if(x!=y) InsertEdge(head,x,y) ;
	}
	Initial() ;
    
	if( dcount[--cnt1] == n )printf("%d",count[cnt1]); 
	else printf("0") ;
	
	return 0 ;
}