hdu-1009- FatMouse' Trade

#include<cstdio>
#include<iostream>
#include<algorithm>
using namespace std;
struct node
{
    int x;
    int y;
    double z;
}a[1010];
int cmp(node a,node b)
{
    return a.z>b.z;
}
int main()
{
    int n,m,i;
    while(cin>>m>>n&&m!=-1&&n!=-1)
    {
        double sum=0.0;
        for(i=0;i<n;i++)
        {
            cin>>a[i].x>>a[i].y;
            a[i].z=a[i].x*1.0/a[i].y;
        }
        sort(a,a+n,cmp);
        for(i=0;i<n;i++)
            if(a[i].y>=m)
        {
            sum+=a[i].x*m*1.0/a[i].y;
            m=0;
        }
        else
        {
            sum+=a[i].x;
            m-=a[i].y;
        }
        printf("%.3lf\n",sum);
    }
    return 0;
}