#include<iostream>
using namespace std;
int main() {
	int n, m;
	cin >> n >> m;
	int* ingred = new int[n + 1];
	int* head = new int[n + 1];
	int* to = new int[m + 1];
	int* w = new int[n + 1];
	int* queue = new int[m + 1];
	int* next = new int[m + 1];
	int* ou = new int[n + 1];
	int l = 0, r = 0;
	for (int i = 0; i <= n; i++) {
		head[i] = -1;
		ingred[i] = 0;
		w[i] = 0;
		ou[i] = 0;
	}
	int a, b;
	for (int i = 1; i <= m; i++) {
		cin >> a >> b;
		to[i] = b;
		ingred[b]++;
		ou[a]++;
		next[i] = head[a];
		head[a] = i;
	}
	for (int i = 1; i <= n; i++) {
		if (ingred[i] == 0&&ou!=0) {
			w[i] = 1;
			queue[r++] = i;
		}
		int ans = 0;
		while (l < r) {
			if (head[queue[l]] == -1) {
				ans += w[queue[l]];
			}
			else {
				for (int ei = head[queue[l]]; ei > 0; ei = next[ei]) {
					if (--ingred[to[ei]] == 0) {
						queue[r++] = to[ei];
					}
					w[to[ei]] += w[queue[l]];
				}
			}
			l++;
		}

		cout << ans << endl;
		return 0;
	}
}

求助