/**************************************************************

     Problem: 1051

     User: Tunix

     Language: C++

     Result: Accepted

     Time:52 ms

     Memory:2308 kb

 ****************************************************************/

 //BZOJ 1051

 #include<vector>

 #include<cstdio>

 #include<cstring>

 #include<cstdlib>

 #include<iostream>

 #include<algorithm>

 #define rep(i,n) for(int i=0;i<n;++i)

 #define F(i,j,n) for(int i=j;i<=n;++i)

 #define D(i,j,n) for(int i=j;i>=n;--i)

 using namespace std;

 const int N=;

 void read(int &v){

     v=;int sign=; char ch=getchar();

     while(ch<'' || ch>'') {if (ch=='-') sign=-; ch=getchar();}

     while(ch>=''&&ch<=''){v=v*+ch-''; ch=getchar();}

     v*=sign;

 }

 /****************tamplate***********************/

 int n,m;

 int head[N],to[N*],next[N*],cnt;

 void add(int a,int b){

     to[++cnt]=b; next[cnt]=head[a]; head[a]=cnt;

 }

 int dfn[N],low[N],belong[N],du[N],dfs_clock,size[N],SCC;

 bool inst[N];

 int st[N],top;

 void tarjan(int x){

     int y;

     dfn[x]=low[x]=++dfs_clock;

     st[top++]=x;

     inst[x]=;

     for(int i=head[x];i;i=next[i]){

         y=to[i];

         if (!dfn[y]){

             tarjan(y);

             low[x]=min(low[x],low[y]);

         }

         else if (inst[y]) low[x]=min(low[x],dfn[y]);

     }

     if (dfn[x]==low[x]){

         SCC++;size[SCC]=;

         for(y=;y!=x;){

             y=st[--top];

             inst[y]=;

             belong[y]=SCC;

             size[SCC]++;

         }

     }

 }

 void rebuild(){

     F(x,,n)

         for(int i=head[x];i;i=next[i])

             if (belong[x]!=belong[to[i]]) du[belong[to[i]]]++;

 }

 /***********************************************/

 int main(){

     read(n); read(m);

     int a,b;

     F(i,,m){

         read(a); read(b);

         add(b,a);

     }

     F(i,,n) if (!dfn[i]) tarjan(i);

     rebuild();

     int ans=;

     if (SCC==) ans=n;

     else{

         int tot=;

         F(i,,SCC) if (du[i]==) {tot++; ans=size[i];}

         if (tot>) ans=;

     }

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

     return ;

 }