用二维数组来做访问标记，别的就是普通的DFS！

#include <iostream>
#include <vector>

using namespace std;

const int N = 1010;
int n, m, u, v;
vector<vector<int>> g(N);
vector<vector<int>> visited(N, vector<int>(N)); // adjacency matrix

void dfs(int cur, int start) {
  if (visited[start][cur]) return;
  visited[start][cur] = 1; // mark as seen
  for (const auto& nxt : g[cur]) dfs(nxt, start);
}

bool check(int x) {
  for (int i = 1; i <= n; i++)
    if (!visited[x][i] && !visited[i][x]) return false;
  return true;
}

int main(void) {
  cin >> n >> m;
  while (m--) {
    cin >> u >> v;
    g[u].emplace_back(v);
  }

  for (int i = 1; i <= n; ++i)
    dfs(i, i);

  int ans = 0;
  for (int i = 1; i <= n; ++i)
    ans += check(i);

  printf("%d\n", ans);
  return 0;
}