建立正向图和反向图，分别沿着两个方向遍历，中途不改变方向，然后查看每个点是否能被其中一个方向遍历的过程中走到

#include <iostream>
#include <cstdio>
#include <cstring>
using namespace std;
const int N = 1e3 + 5, M = 1e4 + 5;

int h[N][2], e[M][2], ne[M][2], cnt[2];
int st[N][2];

void add(int u, int v, int type)
{
    e[++cnt[type]][type] = v;
    ne[cnt[type]][type] = h[u][type];
    h[u][type] = cnt[type];
}

void dfs(int u, int type)
{
    for (int i = h[u][type]; i; i = ne[i][type])
    {
        int v = e[i][type];
        if (!st[v][type])
        {
            st[v][type] = 1;
            dfs(v, type);
        }
    }
}

bool check(int n)
{
    for (int i = 1; i <= n; ++i)
        if (!st[i][0] && !st[i][1]) return false;
    return true;
}

int main()
{
    int n, m;
    scanf("%d%d", &n, &m);
    while (m--)
    {
        int u, v;
        scanf("%d%d", &u, &v);
        add(u, v, 0), add(v, u, 1);
    }

    int ret = 0;
    for (int i = 1; i <= n; ++i)
    {
        memset(st, 0, sizeof st);
        st[i][0] = st[i][1] = 1;
        dfs(i, 0), dfs(i, 1);
        if (check(n)) ret++;
    }
    printf("%d", ret);
    return 0;
}