HDU 1009 FatMouse' Trade

#include <stdio.h>
#include <algorithm>
using namespace std;
struct tt
{
double x,y,z;
} a[10000];
bool cmp(tt x,tt y)
{
return x.z>y.z;
}
int main()
{
int m,n;
while(~scanf("%d%d",&m,&n)&&(m!=-1||n!=-1))
{
double sum = 0;
int i,j;
for(i = 0; i<n; i++)
{
scanf("%lf%lf",&a[i].x,&a[i].y);
a[i].z = a[i].x/a[i].y;
}
sort(a,a+n,cmp);
for(i = 0; i<n; i++)
{
if(m>a[i].y)
{
sum+=a[i].x;
m-=a[i].y;
}
else
{
sum+=a[i].z*m;
break;
}
}
printf("%.3lf\n",sum);
}

return 0;
}