```
#include<iostream>
#include<cstdio>
#include<vector>
#include<stack>
using namespace std;
int n,m,x,y,t,dfn[10001],low[10001],time,v,f[10001],num,d[10001],ans;
bool visit[10001],instack[10001];
vector<int>a[10001];
stack<int>s;
void tarjan(int u){
dfn[u]=low[u]=++time;
visit[u]=1;s.push(u);instack[u]=1;
for(int j=0;j<a[u].size();j++){
v=a[u][j];
if(visit[v]==0){
tarjan(v);
low[u]=min(low[u],low[v]);
}
else if(instack[v]){
low[u]=min(low[u],dfn[v]);
}
}
num=0;
if(dfn[u]==low[u]){
do{
t=s.top();s.pop();instack[t]=0;num++;
}while(t!=u);
if(num>1){
ans++;
}
}
}
int main(){
cin>>n>>m;
for(int i=1;i<=m;i++){
cin>>x>>y;
f[x]=f[y]=1;
a[x].push_back(y);
}
for(int i=1;i<=n;i++){
if(!dfn[i]) tarjan(i);
}
cout<<ans<<endl;
return 0;
}
```
by 九八七六 @ 2018-07-12 17:38:02