HDU1009

#include<stdio.h>
#include<stdlib.h>
struct gg
{
    int j;
    int f;
    double p;
}gl[1005],tp;
int cmp(const void *a,const void *b)
{
    return (*(struct gg*)b).p>(*(struct gg*)a).p?1:-1;
}
int main()
{
    int m,n,i;
    double left,sum;
    while(scanf("%d%d",&m,&n)==2&&(m!=-1)&&(n!=-1))
    {
        for(i=0;i<n;i++)
        {
            scanf("%d%d",&gl[i].j,&gl[i].f);
            gl[i].p=1.0*gl[i].j/gl[i].f;
        }
        qsort(gl,n,sizeof(gl[0]),cmp);
        left=m*1.0;
        sum=0.0;
        for(i=0;i<n&&left>=0;i++)
        {
            if(left>=gl[i].f)
            {
                sum=sum+1.0*gl[i].j;
                left=left-1.0*gl[i].f;
                //if(left==0)break;
            }
            else
                if(left>=0&&left<gl[i].f)
            {
                sum=sum+(gl[i].p)*left;
                left=left-(gl[i].p)*left;
                //left=0;
                //break;
            }
        }
        printf("%.3lf\n",sum);
    }
    return 0;
}