​

 AC代码：

#include <bits/stdc++.h>
using namespace std;
const int N = 100010;
int a[N];

int main() {
	int n, m;
	cin >> n >> m;
	a[0] = 1;
	int cnt = 0;
	for (int i = 0; i < m; i++) {
		int x, y;
		cin >> x >> y;
		if (!a[y])cnt++;
		a[x]=1;
	}
	cout << cnt;
	return 0;
}

​