P1208 [USACO1.3]混合牛奶 Mixing Milk

#include<bits/stdc++.h> 
using namespace std;
struct Node{
	int price;
	int num;
};
bool cmp(Node a,Node b){
	return a.price<b.price;
}
int main()
{
	freopen("in.txt","r",stdin);
	int n,m;cin>>n>>m;vector<Node> ppp;ppp.resize(n);
	for(int i=0;i<n;i++){
		cin>>ppp[i].price>>ppp[i].num;
	}
	sort(ppp.begin(),ppp.end(),cmp);
	int ans=n;int cost=0;
	for(int i=0;i<ppp.size();i++){
		if(ans-ppp[i].num>0){
			ans-=ppp[i].num;
			cost+=ppp[i].num*ppp[i].price;
		}else{
			cost+=ans*ppp[i].price;
			ans=0;break;
		}
	}
	cout<<cost;
	return 0;
}