1 #include <stdio.h>
 2 #include <algorithm>
 3 struct room{int J,F;double P;}a[1010];
 4 bool cmp(const room&a, const room&b){return a.P>b.P;}
 5 
 6 int main()
 7 {
 8     int M, N;
 9     while(scanf("%d%d",&M,&N)&&~M) {
10         for(int i=0; i<N; i++) {
11             scanf("%d%d", &a[i].J, &a[i].F);
12             a[i].P=(double)a[i].J/a[i].F;
13         }
14         std::sort(a,a+N,cmp);
15         double maxf=.0;
16         for(int i=0; i<N; i++)
17             if(M>a[i].F) {
18                 M-=a[i].F;
19                 maxf+=a[i].J;
20             }else{
21                 maxf+=(double)a[i].J * M /a[i].F;
22                 break;
23             }
24         printf("%.3f\n",maxf);
25     }
26     return 0;
27 }