#include<bits/stdc++.h>
using namespace std;
int read(){
int x=0,f=1;char ch=getchar();
while (ch<'0' || ch>'9'){if (ch=='-') f=-1;ch=getchar();}
while (ch>='0' && ch<='9'){x=x*10+ch-'0';ch=getchar();}
return x*f;
}
const int
N=10005,
M=50005;
int n,m,Ecnt,Time,top,num;
int DFN[N],LOW[N],stk[N],NUM[N],sum[N];
int X[M],Y[M],out[N];
bool instack[N],vis[N];
struct Edge{
int next,to;
}E[M<<1];int head[N];
void add(int u,int v){
E[++Ecnt].next=head[u];
E[Ecnt].to=v;
head[u]=Ecnt;
}
void tarjan(int u){
LOW[u]=DFN[u]=++Time;
instack[u]=vis[u]=1;
stk[++top]=u;
for (int i=head[u];i;i=E[i].next){
int j=E[i].to;
if (!vis[j]){
tarjan(j);
LOW[u]=min(LOW[u],LOW[j]);
} else
if (instack[j]) LOW[u]=min(LOW[u],DFN[j]);
}
if (LOW[u]==DFN[u]){
sum[++num]=0;
int now=-1;
for (;now!=u;top--){
now=stk[top];
NUM[now]=num,instack[now]=0,
sum[num]++;
}
}
}
void rebuild(){
Ecnt=0;
memset(head,0,sizeof(head));
for (int i=1;i<=m;i++)
if (NUM[X[i]]!=NUM[Y[i]]) out[NUM[X[i]]]++;
}
int main(){
n=read(),m=read();
Ecnt=0;
for (int i=1;i<=m;i++){
X[i]=read(),Y[i]=read();
add(X[i],Y[i]);
}
num=0;
for (int i=1;i<=n;i++)
if (!vis[i]) top=Time=0,tarjan(i);
if (num==1){printf("%d\n",n);return 0;}
rebuild();
int tmp=0,ans=0;
for (int i=1;i<=num;i++)
if (!out[i]) ans+=sum[i],tmp++;
if (tmp>1) ans=0;
printf("%d\n",ans);
return 0;
}