hdu acm-step 1.3.4 Saving HDU

#include <cstdio>
#include <algorithm>
using namespace std;
struct Node
{
        int pi;
        int mi;
        bool operator<(const Node &node)
        {
                return pi < node.pi;
        }
};
Node a[100];
int main()
{
        int v,n;
        while(scanf("%d%d",&v,&n)&&v)
        {
                for(int i=0;i<n;i++)
                        scanf("%d%d",&a[i].pi,&a[i].mi);
                sort(a,a+n);
                int sum = 0;
                int price = 0;
                for(int i=n-1;i>=0;i--)
                {
                        if(sum+a[i].mi <= v){price+=a[i].pi*a[i].mi;sum+=a[i].mi;}
                        else{price+=(v-sum)*a[i].pi;sum=v;}
                        if(sum == v)break;
                }
                printf("%d\n",price);
        }
        return 0;
}