提交后4个AC3个WA1个RE，本人太蒻，调不出来。

#include<iostream>
#include<algorithm>
using namespace std;
int n, m, p[5005], a[5005], sum, ans;
int main() {
	ios::sync_with_stdio(false);
	cin.tie(0);
	cout.tie(0);
	cin >> n >> m;
	for (int i = 1; i <= m; i++) cin >> p[i] >> a[i];
	sort(p + 1, p + m + 1);
	for (int i = 1; sum < n; i++) {
		if (sum + a[i] <= n) {
			sum += a[i];
			ans += p[i] * a[i];
		}
		else {
			ans += (n - sum) * p[i];
		}
	}
	cout << ans;
	return 0;
}