1 #include<cstdio>
2 #include<algorithm>
3 #include<cstring>
4 #include<vector>
5 #include<stack>
6 #define M 50010
7 #define N 10010
8 using namespace std;
9 int n,m,u,v,head[N],cnt=1,ans,out[N],belong[N],dfn[N],low[N],indx,tar,miao;
10 bool inst[N];
11 stack <int> st;
12 struct edge
13 {
14 int u,v;
15 }e[M],tmp[M];
16 void add(int u,int v)
17 {
18 e[cnt].v=v;
19 e[cnt].u=head[u];
20 head[u]=cnt++;
21 }
22 void dfs(int u)
23 {
24 dfn[u]=low[u]=++indx;
25 inst[u]=1;
26 st.push(u);
27 for (int i=head[u];i;i=e[i].u)
28 {
29 int v=e[i].v;
30 if(!dfn[v])
31 {
32 dfs(v);
33 low[u]=min(low[u],low[v]);
34 }
35 else
36 if (inst[v])
37 low[u]=min(low[u],dfn[v]);
38 }
39 if (dfn[u]==low[u])
40 {
41 tar++;
42 while (1)
43 {
44 int t=st.top();
45 st.pop(),inst[t]=0;
46 belong[t]=tar;
47 if (t==u)
48 break;
49 }
50 }}
51 int main()
52 {
53 scanf("%d%d",&n,&m);
54 for (int i=1;i<=m;i++)
55 {
56 scanf("%d%d",&u,&v);
57 add(u,v);
58 tmp[i].u=u;
59 tmp[i].v=v;
60 }
61 for (int i=1;i<=n;i++)
62 if (dfn[i]==0) dfs(i);
63 for (int i=1;i<=m;i++)
64 {
65 u=tmp[i].u,v=tmp[i].v;
66 if (belong[u]!=belong[v])
67 ++out[belong[u]];
68 }
69 for (int i=1;i<=tar;i++)
70 if (out[i]==0)
71 if (miao==0)
72 miao=i;
73 else miao=-1;
74 for (int i=1;i<=n;i++)
75 if (belong[i]==miao) ans++;
76 printf("%d\n",ans);
77 return 0;
78 }
