zoj 2109 FatMouse' Trade

#include<stdio.h>
#include<string.h>
#include<iostream>
#include<algorithm>
#include<math.h>
using namespace std;
struct data
{
double j,f,x;
}a[1024];
bool cmp(data a,data b)
{
return a.x>b.x;
}

int main()
{
int m,n,i;
double sum;
while(~scanf("%d %d",&m,&n))
{
if(m==-1&&n==-1) break;
for(i=0;i<n;i++)
{
scanf("%lf%lf",&a[i].j,&a[i].f);
a[i].x=a[i].j/a[i].f;
}
sum=0;
sort(a,a+n,cmp);
for(i=0;i<n;i++)
{
if(m>a[i].f)
{
sum+=a[i].j;
m-=a[i].f;
}
else
{
sum+=m*a[i].x;
m=0;
break;
}
}
printf("%.3f\n",sum);
}
return 0;
}