/**************************************************************    Problem: 1051    User: BeiYu    Language: C++    Result: Accepted    Time:76 ms    Memory:3048 kb****************************************************************/ #include<cstdio>#include<stack>#include<vector>#include<iostream>using namespace std; const int N = 10005;#define debug(a) cout<<#a<<"="<<a<<" "#define ct cout<<endl#define _ct cout<<"----------"<<endl int n,m,cnt,bcnt;int dfsn[N],low[N],ins[N],b[N],sz[N];vector<int> g[N];vector<int> h[N];stack<int> stk;struct Edge{ int fr,to; }edge[N*5]; inline int in(int x=0,char ch=getchar()){ while(ch>‘9‘||ch<‘0‘) ch=getchar();    while(ch>=‘0‘&&ch<=‘9‘) x=(x<<3)+(x<<1)+ch-‘0‘,ch=getchar();return x; }void Tarjan(int u,int fa){    dfsn[u]=low[u]=++cnt,ins[u]=1,stk.push(u);    for(int i=0,lim=g[u].size(),v;i<lim;i++) if((v=g[u][i])!=fa){        if(!dfsn[v]){ Tarjan(v,u),low[u]=min(low[u],low[v]); }        else if(ins[v]) low[u]=min(low[u],dfsn[v]);    }if(dfsn[u]==low[u]){        ++bcnt;int v;        for(;;){ v=stk.top(),stk.pop(),ins[v]=0,b[v]=bcnt,sz[bcnt]++;if(u==v) break; }    }}int main(){//  freopen("in.in","r",stdin);    n=in(),m=in();    for(int i=1,u,v;i<=m;i++) u=in(),v=in(),g[u].push_back(v),edge[i]=(Edge){ u,v };    for(int i=1;i<=n;i++) if(!dfsn[i]) Tarjan(i,0);//  _ct;debug(bcnt),ct;    for(int i=1,u,v;i<=m;i++){        u=edge[i].fr,v=edge[i].to;//      debug(u),debug(v),ct;//      debug(b[u]),debug(b[v]),ct;        if(b[u]!=b[v]) h[b[u]].push_back(b[v]);    }//  _ct;    int tmp=0,ans=0;    for(int i=1;i<=bcnt;i++){        if(h[i].size()==0) ans=sz[i],tmp++;    }    if(tmp!=1) putchar(‘0‘);else printf("%d",ans);    return 0;}