#include <iostream>

  #include<cstdio>

  #include<cstring>

  #include<algorithm>

  #include<stdlib.h>

  #include<stack>

  using namespace std;

  #define M 50010

  #define N 10010

  struct node

  {

      int u,v,next;

  }edge[M*];

  int head[N],lowlink[N],pre[N],t,sccno[N],scc,dep,dout[N];

  stack<int>s;

  void init()

  {

      t = ;

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

      memset(lowlink,,sizeof(lowlink));

      memset(pre,,sizeof(pre));

      memset(sccno,,sizeof(sccno));

      dep=;scc=;

  }

  void add(int u,int v)

  {

      edge[t].u = v;

      edge[t].v = v;

      edge[t].next = head[u];

      head[u] = t++;

  }

  void dfs(int u)

  {

      lowlink[u] = pre[u] = ++dep;

      s.push(u);

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

      {

          int v = edge[i].v;

          if(!pre[v])

          {

              dfs(v);

              lowlink[u] = min(lowlink[u],lowlink[v]);

          }

          else if(!sccno[v])

          lowlink[u] = min(lowlink[u],pre[v]);

      }

      if(lowlink[u]==pre[u])

      {

          scc++;

          for(;;)

          {

              int x = s.top();s.pop();

              sccno[x] = scc;

              if(x==u) break;

          }

      }

  }

  void find_scc(int n)

  {

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

      if(!pre[i]) dfs(i);

  }

  int main()

  {

      int i,j,n,m,a,b;

      while(cin>>n>>m)

      {

          init();

          while(m--)

          {

              cin>>a>>b;

              add(a,b);

          }

          find_scc(n);

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

          dout[i] = ;

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

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

              {

                  int v = edge[j].v;

                  if(sccno[i]!=sccno[v])

                      dout[sccno[i]] = ;

              }

          int num=,o,w=;

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

              if(dout[i]==)

              {

                  o = i;

                  num++;

              }

          if(num==)

          cout<<n<<endl;

          else if(num==)

          {

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

              if(sccno[i]==o)

              {

                 w++;

              }

              cout<<w<<endl;

          }

          else

          cout<<"0\n";

      }

      return ;

  }