1 #include <iostream>
 2 #include <algorithm>
 3 #include <cstdio>
 4 using namespace std;
 5 typedef struct {
 6     int j, f;
 7     double aver;
 8 }food;
 9 
10 bool cmp(food a, food b){
11     return a.aver >= b.aver;
12 }
13 
14 food javabean[1005];
15 int main(){
16     int m, n;
17     while(scanf("%d%d", &m, &n) != EOF){
18         if((m == -1) && (n == -1))
19             break;
20         
21         for(int i = 0; i < n; i++){
22             scanf("%d %d", &javabean[i].j, &javabean[i].f);
23             javabean[i].aver = (double)javabean[i].j / javabean[i].f;
24         }
25         sort(javabean, javabean+n, cmp);
26         double sum = 0;
27         for(int i = 0; i < n; i++){
28             if(m >= javabean[i].f){
29                 sum += javabean[i].j;
30                 m -= javabean[i].f;
31             }
32             else {
33                 sum += (double)javabean[i].j / javabean[i].f * m;
34                 break;
35             }
36         }
37         printf("%.3lf\n", sum);
38     }
39     return 0;
40 }