1 #include<cstring>
 2 #include<cstdio>
 3 #include<cmath>
 4 #include<algorithm>
 5 #include<cctype>
 6 #define foru(i,x,y) for(int i=x;i<=y;i++)
 7 #define ford(i,x,y) for(int i=x;i>=y;i--)
 8 #define re(x) x=read()
 9 using namespace std;
10 typedef long long LL;
11 typedef double db;
12 const int inf=1e9;
13 const int N=1e6+10;
14
15 struct edge{int to,nxt;}e[N*2];
16 int dfn[N],low[N],stk[N],head[N],vis[N],bl[N],out[N],x,cnt,cl,ed,ne,n,m,ans;
17
18 void add(int a,int b){
19     e[++ne]=(edge){b,head[a]};head[a]=ne;
20 }
21
22 int read(){
23     static int f,x;static char ch;
24     x=f=0;ch=getchar();
25     while(!isdigit(ch)){f=(ch=='-');ch=getchar();}
26     while(isdigit(ch)){x=x*10+ch-'0';ch=getchar();}
27     return f?-x:x;
28 }
29
30 void tarjan(int k){
31     dfn[k]=low[k]=++cnt;stk[++ed]=k;vis[k]=1;
32     for(int i=head[k];i;i=e[i].nxt){
33         int v=e[i].to;
34         if(!dfn[v]){
35             tarjan(v);
36             low[k]=min(low[k],low[v]);
37         }
38         else if(vis[v])low[k]=min(low[k],dfn[v]);
39     }
40     int p;
41     if(low[k]==dfn[k]){
42         cl++;//x++;
43         do{
44             p=stk[ed--];
45             bl[p]=cl;
46             vis[p]=0;
47         }while(p!=k);
48     }
49 }
50
51 int main(){
52     int u,v;
53     re(n);re(m);
54     foru(i,1,m){
55         re(u);re(v);
56         add(u,v);
57     }
58     foru(i,1,n)if(!dfn[i])tarjan(i);
59     if(cl==1){printf("%d\n",n);return 0;}
60     foru(k,1,n)
61         for(int i=head[k];i;i=e[i].nxt)
62             if(bl[k]!=bl[e[i].to])out[bl[k]]++;
63     v=0;
64     foru(i,1,cl)if(!out[i]){v++;u=i;}
65     if(v==1){
66         foru(i,1,n)if(bl[i]==u)ans++;
67         printf("%d\n",ans);return 0;
68     }
69     else puts("0");
70
71 //    if(v==1)
72 //        foru(i,1,n)if(bl[i]==u)ans++;
73 //    else ans=0;
74 //    printf("%d\n",ans);
75     return 0;
76 }