#include<cstdio>
using namespace std;
int n, m;
int a[100], b[100];
int main() {
	scanf("%d%d", &n, &m);
	for (int i = 1; i <= n; ++i)
		a[i] = b[i] = 0;
	for (int i = 1; i <= m; ++i) {
		int x, y;
		scanf("%d%d", &x, &y);
		if (a[x] < y && b[y] < x) {
			if (a[x] > 0)
				b[a[x]] = 0;
			if (b[y] > 0)
				a[b[y]] = 0;
			a[x] = y;
			b[y] = x;
		}
	}
	int ans = 0;
	for (int i = 1; i <= n; ++i) {
		if (a[i] == 0)
			++ans;
		if (b[i] == 0)
			++ans;
	}
	printf("%d", ans);
	return 0;
}

大佬们，这个代码是什么意思