USACO 1.3 Mixing Milk

/*
ID: aznfy1
PROG: milk
LANG: C++
*/

#include<stdio.h>
#include<string.h>
#include<iostream>
#include<algorithm>
#define clr(a,b); memset(a,b,sizeof(a));
using namespace std;


struct node
{
    int price;
    int have;
}a[6000];
int need;
int n;
int money;
bool cmp(const node& a,const node& b)
{
    return a.price<b.price;
}
int main()
{
    freopen("milk.in","r",stdin);
    freopen("milk.out","w",stdout);
    while(cin>>need>>n)
    {
        if(need==0&&n==0)
        {cout<<0<<endl;continue;}
        money=0;
        for(int i=0;i<n;i++)
        {
            cin>>a[i].price>>a[i].have;
        }
        sort(a,a+n,cmp);
        for(int i=0;i<n;i++)
        {
            if(a[i].have<=need)
            {
                need-=a[i].have;
                money+=a[i].have*a[i].price;
            }
            else
            {
                money+=a[i].price*need;
                break;
            }
        }
        cout<<money<<endl;
    }
}

太水