1 #include<iostream>
 2 #include<cstdio>
 3 #include<algorithm>
 4 #include<iomanip>
 5 using namespace std;
 6 #define N 1005
 7 
 8 struct Trade{
 9     int J, F;       //J代表JavaBeans，F代表cat food
10     double price;   //性价比
11 }trade[N];
12 
13 int cmp(Trade a,Trade b)
14 {
15     return a.price>b.price;
16 }
17 int main()
18 {
19     double maximum;
20     int m, n, i;
21     while(cin>>m>>n)
22     {
23         maximum = 0;
24         if(m==-1 && n==-1)
25             break;
26         maximum = 0;
27         for(i=0; i<n; i++)
28         {
29             cin>>trade[i].J>>trade[i].F;
30             trade[i].price = trade[i].J*1.0/trade[i].F;
31         }
32         sort(trade,trade+n,cmp);
33         for(i=0; i<n; i++)
34         {
35             if(m == 0)
36                 break;
37             else if(trade[i].F<=m)
38             {
39                 maximum += trade[i].J;
40                 m -= trade[i].F;
41             }
42             else
43             {
44                 maximum += m*trade[i].price;
45                 m = 0;
46             }
47         }
48         printf("%.3lf\n",maximum);
49     }
50     return 0;
51 }