1 #include <cstdio> 2 int n,m,p,q,N=0,time=0,T=0,sum=0,ans=0; 3 int from[50001],to[50001],nex[50001],fir[10001],dfn[10001],low[10001],l[10001],tar[10001],gui[10001],c[10001]; 4 bool que[10001]; 5 void add(int p,int q){    from[++N]=p,to[N]=q,nex[N]=fir[p],fir[p]=N;} 6 int min(int p,int q){    return (p<q)?p:q;} 7 void dfs(int k) 8 { 9     dfn[k]=low[k]=++time,l[++T]=k,que[k]=1;10     for(int o=fir[k];o;o=nex[o])11         if(!dfn[to[o]])12             dfs(to[o]),low[k]=min(low[k],low[to[o]]);13         else14             if(que[to[o]])15                 low[k]=min(low[k],dfn[to[o]]);16     if(dfn[k]==low[k])17     {18         sum++;19         while(l[T]!=k)20             que[l[T]]=0,tar[l[T--]]=sum,gui[sum]++;21         que[k]=0;tar[k]=sum;T--;gui[sum]++;22     }23 }24 int main()25 {26     scanf("%d%d",&n,&m);27     for(int i=1;i<=m;i++)28         scanf("%d%d",&p,&q),add(p,q);29     for(int i=1;i<=n;i++)30         if(!dfn[i]) dfs(i);31     for(int i=1;i<=N;i++)32     if(tar[from[i]]!=tar[to[i]])33         c[tar[from[i]]]=1;34     for(int i=1;i<=sum;i++)35         if(!c[i])36             if(ans){    printf("0\n");return 0;}37             else ans=gui[i];38     printf("%d\n",ans);39     return 0;40 }