#include <stack>
#include <cstdio>
#include <vector>
#include <algorithm>
using namespace std;

using ll=long long;
const int Inf=0x3f3f3f3f;
const int mod=1e9+7;
const int N=1e4+5;

vector<int> G[N];
stack<int> s;
int dfn[N],low[N],sccno[N],out[N],sum[N],scc_cnt,ck;

void tarjan(int u)
{
	dfn[u]=low[u]=++ck;
	s.push(u);
	for(auto v:G[u])
		if(!dfn[v])
		{
			tarjan(v);
			low[u]=min(low[u],low[v]);
		}
		else if(!low[v])
			low[u]=min(low[u],dfn[v]);
	if(low[u]==dfn[u])
	{
		++scc_cnt;
		while(1)
		{
			int t=s.top();
			s.pop();
			sccno[t]=scc_cnt;
			sum[scc_cnt]++;
			if(t==u)break;
		}
	}
}
int main()
{
	int n,m;
	scanf("%d%d",&n,&m);
	while(m--)
	{
		int u,v;
		scanf("%d%d",&u,&v);
		G[u].push_back(v);
	}
	for(int i=1;i<=n;i++)
		if(!dfn[i])
			tarjan(i);
	for(int u=1;u<=n;u++)
		for(auto v:G[u])
			if(sccno[u]!=sccno[v])
				out[sccno[u]]++;
	int ans=0;
	for(int i=1;i<=scc_cnt;i++)
		if(out[i]==0)
		{
			if(ans)
			{
				printf("0\n");
				return 0;
			}
			ans=i;
		}
	printf("%d\n",sum[ans]);
}

附题目传送门与测评结果