1 /************************************************************** 2     Problem: 1654 3     User: rausen 4     Language: C++ 5     Result: Accepted 6     Time:32 ms 7     Memory:1476 kb 8 ****************************************************************/ 9  10 #include <cstdio>11 #include <cstring>12 #include <algorithm>13  14 using namespace std;15 const int N = 10005;16 const int M = 50005;17 struct edges{18     int next, to;19 }e[M];20 int n, m;21 int cnt, top, num, tot, ans, sz;22 int dfn[N], low[N], vis[N], s[N], first[N];23  24 inline int read(){25     int x = 0, sgn = 1;26     char ch = getchar();27     while (ch < ‘0‘ || ch > ‘9‘){28         if (ch == ‘-‘) sgn = -1;29         ch = getchar();30     }31     while (ch >= ‘0‘ && ch <= ‘9‘){32         x = x * 10 + ch - ‘0‘;33         ch = getchar();34     }35     return sgn * x;36 }37  38 inline void add_edge(int x, int y){39     e[++tot].next = first[x];40     first[x] = tot;41     e[tot].to = y;42 }43  44 void DFS(int p){45     dfn[p] = low[p] = ++cnt;46     s[++top] = p, vis[p] = 1;47     int x, y;48     for (x = first[p]; x; x = e[x].next){49         y = e[x].to;50         if (!vis[y]) DFS(y);51         if (vis[y] < 2)52             low[p] = min(low[p], low[y]);53     }54     if (dfn[p] == low[p]){55         ++num, sz = 0;56         while (s[top + 1] != p){57             y = s[top--];58             vis[y] = 2;59             ++sz;60         }61         if (sz != 1) ++ ans;62     }63 }64  65 inline void tarjan(){66     cnt = top = num = 0;67     memset(vis, sizeof(vis), 0);68     for (int i = 1; i <= n; ++i)69         if (!vis[i]) DFS(i);70 }71  72 int main(){73     n = read(), m = read();74     int x, y;75     for (int i = 1; i <= m; ++i){76         x = read(), y = read();77         add_edge(x, y);78     }79     tarjan();80     printf("%d\n", ans);81     return 0;82 }