#include<bits/stdc++.h>
#define ll long long
using namespace std;

ll read(){
	ll X = 0 ; int f = 1;
	char ch = getchar();
	while(!isdigit(ch) && ch != '-') ch = getchar();
	if(ch == '-'){
		ch = getchar();
		f = -1;
	}
	while(isdigit(ch)){
		X = (X<<1)+(X<<3)+ch-'0';
		ch = getchar();
	}
	return X*f;
}

const int N = 1e4+10 , M = 5e4+10;
int n,m; 

int to[M] , nxt[M] , head[N] , tot;
inline void add(int x,int y){
	to[++tot] = y;
	nxt[tot] = head[x];
	head[x] = tot;
}

int dfn[N],low[N],c[N],Num,Sum,ans[N];
int s[N],top; bool ins[N];
void tarjan(int x){
	dfn[x] = low[x] = ++Num;
	s[++top] = x; ins[x] = true;
	for(int i=head[x];i;i=nxt[i]){
		int y = to[i];
		if(!dfn[y]){
			tarjan(y);
			low[x] = min(low[x],low[y]);
		}else if(!ins[y])
			low[x] = min(low[x],dfn[y]);
	}
	if(dfn[x] == low[x]){
		++Sum;
		int k;
		do{
			k = s[top]; top--;
			ins[k] = false;
			c[k] = Sum;
			ans[Sum]++;
		}while(k != x);
	}
}
int out[N];
int main(){
	n = read(); m = read();
	for(int i=1;i<=m;i++){
		int a = read() , b = read();
		add(a,b);
	}
	for(int i=1;i<=n;i++)
		if(!dfn[i]) tarjan(i);
	for(int u=1;u<=n;u++)
		for(int j=head[u];j;j=nxt[j]){
			int v = to[j];
			if(c[u] != c[v]) out[c[u]]++;
		}
	int cnt = 0;
	for(int i=1;i<=Sum;i++)
		if(!out[i]){
			if(cnt){
				cout << 0;
				return 0;
			}
			cnt = i;
		}
	cout << ans[cnt];
	return 0;
}