1 #include <stack> 2 #include <cstdio> 3 #include <cstring> 4 #include <iostream> 5  6 const int MAXN = 50001; 7 int n, m, cnt, idx, size, ans; 8 int head[MAXN], to[MAXN << 1], next[MAXN << 1]; 9 int dfn[MAXN], low[MAXN], belong[MAXN], tot[MAXN];10 bool ins[MAXN];11 std::stack <int> s;12 13 inline int read()14 {15     int x = 0, f = 1;16     char ch = getchar();17     for(; !isdigit(ch); ch = getchar()) if(ch == ‘-‘) f = -1;18     for(; isdigit(ch); ch = getchar()) x = (x << 1) + (x << 3) + ch - ‘0‘;19     return x * f;20 }21 22 inline int min(int x, int y)23 {24     return x < y ? x : y;25 }26 27 inline void add(int x, int y)28 {29     to[cnt] = y;30     next[cnt] = head[x];31     head[x] = cnt++;32 }33 34 inline void dfs(int u)35 {36     int i, v;37     s.push(u);38     ins[u] = 1;39     dfn[u] = low[u] = ++idx;40     for(i = head[u]; i ^ -1; i = next[i])41     {42         v = to[i];43         if(!dfn[v])44         {45             dfs(v);46             low[u] = min(low[u], low[v]);47         }48         else if(ins[v]) low[u] = min(low[u], dfn[v]);49     }50     if(low[u] == dfn[u])51     {52         size++;53         do54         {55             v = s.top();56             s.pop();57             ins[v] = 0;58             belong[v] = size;59         }60         while(u ^ v);61     }62 }63 64 int main()65 {66     int i, x, y;67     n = read();68     m = read();69     memset(head, -1, sizeof(head));70     for(i = 1; i <= m; i++)71     {72         x = read();73         y = read();74         add(x, y);75     }76     for(i = 1; i <= n; i++)77         if(!dfn[i])78             dfs(i);79     for(i = 1; i <= n; i++) tot[belong[i]]++;80     for(i = 1; i <= size; i++)81         if(tot[i] > 1)82             ans++;83     printf("%d\n", ans);84     return 0;85 }