tarjan

#include<iostream>
#include<cstring>
#include<cstdio>
#include<algorithm>
#include<vector>
using namespace std;
const int M=1e4+5;
vector<int>g[M];
int out[M],cmp[M],dfn[M],low[M],sta[M],vis[M],sum[M],tot,cnt,top;
void tarjan(int u){
    dfn[u]=low[u]=++cnt;
    sta[++top]=u;
    vis[u]=true;
    for(int i=0;i<g[u].size();i++){
        int v=g[u][i];
        if(!dfn[v]){
            tarjan(v),low[u]=min(low[u],low[v]);
        }
        else if(vis[v])
            low[u]=min(low[u],dfn[v]);
    }
    if(dfn[u]==low[u]){
        cmp[u]=++tot;
        vis[u]=false;
        int len=top;
        while(sta[top]!=u){
            cmp[sta[top]]=tot;
            vis[sta[top--]]=false;
        }
        top--;
        sum[tot]=len-top;
    }
}
int main(){
    int n,m;
    scanf("%d%d",&n,&m);
    while(m--){
        int x,y;
        scanf("%d%d",&x,&y);
        g[x].push_back(y);
    }
    for(int i=1;i<=n;i++)
        if(!dfn[i])
            tarjan(i);
    for(int i=1;i<=n;i++){
        for(int j=0;j<g[i].size();j++){
            int v=g[i][j];
            if(cmp[i]!=cmp[v])
                out[cmp[i]]++;
        }
    }
    /*for(int i=1;i<=tot;i++)
        cout<<out[i]<<" ";
    cout<<endl;*/
    int s=0;
    for(int i=1;i<=tot;i++){
        if(!out[i]){
            if(s){
                return puts("0"),0;
            }
            else
                s=sum[i];
        }
    }
    printf("%d\n",s);
    return 0;
}