#include <iostream>

 #include <vector>

 #include <queue>

 #include <string.h>

 #include <stdlib.h>

 #include <stdio.h>

 #include <stack>

 using namespace std;

 const int maxn = ;

 struct edge

 {

     int u,v,val;

 };

 int dfn[],low[],belong[],inst[];

 vector<edge> edges;

 vector<int>g[maxn];

 stack<int>st;

 int bcnt,cnt,time;

 void init(int n)

 {

     int i;

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

     g[i].clear();

     edges.clear();

     time = ;bcnt = cnt = ;

     return ;

 }

 void addedge(int u,int v,int val)

 {

     edges.push_back((edge){u,v,});

     g[u].push_back(cnt);

     cnt++;

     return ;

 }

 void tarjan(int i)

 {

     int j;

     dfn[i] = low[i] = ++time;

     inst[i] = ;

     st.push(i);

     for(j = ;j < g[i].size();j++)

     {

         edge e;

         e = edges[g[i][j]];

         int v;

         v = e.v;

         if(!dfn[v])

         {

             tarjan(v);

             low[i] = min(low[i],low[v]);

         }

         else if(inst[v] && dfn[v] < low[i])

         low[i] = dfn[v];

     }

     if(dfn[i] == low[i])

     {

         //cout<<i<<"****"<<endl;

         bcnt++;

         do

         {

             j = st.top();

             st.pop();

             inst[j] = ;

             belong[j] = bcnt;

             //printf("%d *",j);

         }

         while(j != i);

         //puts("");

     }

 }

 void tarjans(int n)

 {

     int i;

     bcnt = time = ;

     while(!st.empty())st.pop();

     memset(dfn,,sizeof(dfn));

     memset(inst,,sizeof(inst));

     memset(belong,,sizeof(belong));

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

     if(!dfn[i])tarjan(i);

 }

 int main()

 {

     int n,m;

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

     while(~scanf("%d %d",&n,&m))

     {

         int a,b,i;

         init(n);

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

         {

             scanf("%d %d",&a,&b);

             addedge(a,b,);

         }

         tarjans(n);

         //cout<<bcnt<<endl;

         int leap = ,flag = ;

         int bcnt0,j;

         int outbc[];

         memset(outbc,,sizeof(outbc));

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

         {

             for(j = ;j < g[i].size();j++)

             {

                 int v;

                 v = edges[g[i][j]].v;

                 if(belong[i] != belong[v]){

                 if(outbc[belong[i]] == )

                 leap++;

                 outbc[belong[i]] = ;

                 }

             }

         }

         if(leap == bcnt-)

         {

             int ans;

             ans = ;

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

             {

                 if(outbc[i] == )

                 break;

             }

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

             if(belong[j] == i)

                 ans++;

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

         }

         else

         puts("");

     }

     return ;

 }