luogu P1208 [USACO1.3]混合牛奶 Mixing Milk

P1208

#include<bits/stdc++.h>
using namespace std;
typedef long long ll;
#define INF 0x3f3f3f3f
struct group{
    
    
	int p,a;
}t[5009];
bool cmp(group a,group b){
    
    
	return a.p<b.p;
}
int main(){
    
    
	int n,m;
	cin>>n>>m;
	if(n==0){
    
    
		cout<<"0";
		return 0;
	}
	 for(int i=0;i<m;i++)cin>>t[i].p>>t[i].a;
	sort(t,t+m,cmp);
	int sum=0,ans=0;
	for(int i=0;i<m;i++){
    
    
		if(sum+t[i].a<n)ans+=t[i].a*t[i].p,sum+=t[i].a;
		else {
    
    
			ans+=(n-sum)*t[i].p;
			cout<<ans;
			return 0;
		}
	}
	return 0;
	
}