1 #include <iostream>
2 #include <stdio.h>
3 #include <cstring>
4 #include <cmath>
5 #include <algorithm>
6
7
8 using namespace std;
9
10 struct node
11 {
12 double J; // JavaBean
13 double F; // Food
14 double rate; // 性价比
15 }arr[1000];
16
17 bool cmp(node a, node b)
18 {
19 return a.rate > b.rate;
20 }
21
22 int main()
23 {
24 int m, n;
25 double a, b;
26 while(cin >> m >> n)
27 {
28 if(m == -1 && n == -1)
29 break;
30
31 for(int i = 0; i < n; ++i)
32 {
33 cin >> arr[i].J >> arr[i].F;
34 arr[i].rate = (arr[i].J) / (arr[i].F);
35 }
36
37 sort(arr, arr+n, cmp);
38
39 double ans = 0;
40 for(int i = 0; i < n; ++i)
41 {
42 if(m > arr[i].F)
43 {
44 ans += arr[i].J;
45 m -= arr[i].F;
46 }
47 else
48 {
49 ans += m*arr[i].rate;
50 break;
51 }
52
53 }
54
55 printf("%0.3f\n", ans);
56
57 }
58
59
60 return 0;
61 }
