P1208 [USACO1.3] 混合牛奶 Mixing Milk

#include<bits/stdc++.h> 
using namespace std;
int n,m;
long long sum;
struct ss{
	int x;
	int y;
}a[5005];
bool cmp(ss a,ss b){
	if(a.x!=b.x)return a.x<=b.x;
	else return a.y>b.y;
}
int main(){
	cin>>n>>m;
	if(n==0||m==0){
		cout<<0;
		return 0;
	}
	for(int i=0;i<m;i++)cin>>a[i].x>>a[i].y;
	sort(a,a+n,cmp);
	for(int i=0;i<m;i++){
		if(n>=a[i].y){
			n-=a[i].y;
			sum+=(a[i].x*a[i].y);
		}
		else{
			sum+=(a[i].x*n);
			break;
		}
	}
	cout<<sum;
	return 0;
}