#include<stdio.h>
#include<iostream>
#include<vector>
using namespace std;
int Ctc,n,m,id,st[10001],ctd[10001],ict[10001],ist[10001],top,DFN[10001],LOW[10001];
vector<int> Gr[10001];
vector<int> Ct[10001];
void tarjan(int i)
{int j,k;
 ist[i]=1;
 st[++top]=i;
 DFN[i]=LOW[i]=id++;
 for(k=0;k<Gr[i].size();k++)
 {j=Gr[i][k];
  if(DFN[j]==-1)
  {tarjan(j);
   LOW[i]=min(LOW[i],LOW[j]);
  }
  else if(ist[j]) LOW[i]=min(LOW[i],DFN[j]);
 }
 if(LOW[i]==DFN[i])
 {Ctc++;
  do
  {j=st[top--];
   ist[j]=0;
   ict[j]=Ctc;
   Ct[Ctc].push_back(j);
  }while(j!=i);
 }
}
int main()
{freopen("poj2186.in","r",stdin);
 freopen("poj2186.out","w",stdout);
 cin>>n>>m;
 int i,j,x,y,k,loc,t=0;
 for(i=1;i<=m;i++)
 {scanf("%d%d",&x,&y);
  Gr[x].push_back(y);
 }
 memset(DFN,-1,sizeof(DFN));
 memset(LOW,-1,sizeof(LOW));
 for(i=1;i<=n;i++)
  if(DFN[i]==-1) tarjan(i);
 for(i=1;i<=n;i++)
 {for(k=0;k<Gr[i].size();k++)
  {j=Gr[i][k];
   if(ict[i]!=ict[j]) ctd[ict[i]]++;
  }
 }
 for(i=1;i<=Ctc;i++)
  if(ctd[i]==0){t++;loc=i;}
 if(t==1) printf("%d",Ct[loc].size());
 else cout<<0;
 return 0;
}