#include<cstdio>

 #include<iostream>

 #include<cstring>

 const int MAXN=;//点数

 const int MAXM=;//边数

 struct Edge

 {

     int to,next;

 }edge[MAXM];

 int head[MAXN],tot;

 int Low[MAXN],DFN[MAXN],Stack[MAXN],Belong[MAXN];//Belong数组的值是1~scc

 int Index,top;

 int scc;//强连通分量的个数

 bool Instack[MAXN];

 int num[MAXN];//各个强连通分量包含点的个数，数组编号1~scc

 //num数组不一定需要，结合实际情况

 int out[MAXN],tmp,Num,ans;

 void addedge(int u,int v)

 {

     edge[tot].to=v;edge[tot].next=head[u];head[u]=tot++;

 }

 void Tarjan(int u)

 {

     int v;

     Low[u]=DFN[u]=++Index;

     Stack[top++]=u;

     Instack[u]=true;

     for(int i=head[u];i != -;i=edge[i].next)

     {

         v=edge[i].to;

         if(!DFN[v])

         {

             Tarjan(v);

             if(Low[u] > Low[v])Low[u]=Low[v];

         }

         else if(Instack[v] && Low[u] > DFN[v])

         Low[u]=DFN[v];

     }

     if(Low[u]==DFN[u])

     {

         scc++;

         do

         {

             v=Stack[--top];

             Instack[v]=false;

             Belong[v]=scc;

             num[scc]++;

         }

         while(v != u);

     }

 }

 void solve(int N)

 {

     memset(out,,sizeof(out));

     memset(Belong,,sizeof(Belong));

     memset(DFN,,sizeof(DFN));

     memset(Instack,false,sizeof(Instack));

     memset(num,,sizeof(num));

     Index=scc=top=;

     for(int i=;i <= N;i++)

         if(!DFN[i])

             Tarjan(i);

 }

 void init()

 {

     tot=;

     memset(head,-,sizeof(head));

 }

 int main()

 {

     int n,m;

     int i,j,v;

     //freopen("1.in","r",stdin);

     while(scanf("%d%d",&n,&m)!=EOF)

     {

         init();

         int q,p;

         for(i=;i<=m;i++)

         {

             scanf("%d%d",&p,&q);

             addedge(p,q);

         }

         solve(n);

         for(i=;i<=n;i++)

         {

             for(v=head[i];v!=-;v=edge[v].next)

             {

                 if(Belong[i]!=Belong[edge[v].to])

                 {

                     out[Belong[i]]++;

                 }

             }

         }

         ans=,Num=;

         for(i=;i<=scc;i++)

         {

             if(!out[i])

             {

                 Num++;

                 tmp = i;

             }

         }

         if(Num==)

         {

             for(i=;i<=n;i++)

             {

                 if(Belong[i]==tmp)

                     ans++;

             }

             printf("%d\n",ans);

         }

         else

         {

             printf("0\n");

         }

     }

     return ;

 }