1 #include <cstdio>
 2 #include <algorithm>
 3 using namespace std;
 4 
 5 const int MAX=1050;
 6 struct Room{
 7     double j;
 8     double f;
 9     double w;
10 }r[MAX];
11 
12 bool compare(const Room &r1,const Room &r2)
13 {
14     if(r1.w>r2.w)
15         return true;
16     else
17         return false;
18 }
19 
20 int main()
21 {
22     double m,ans,jv,fv;
23     int n;
24     while(scanf("%lf%d",&m,&n)==2&&n!=-1)
25     {
26         ans=0;
27         for(int i=0;i<n;i++)
28         {
29             scanf("%lf%lf",&jv,&fv);
30             r[i].j=jv;
31             r[i].f=fv;
32             r[i].w=jv/fv;
33         }
34         sort(r,r+n,compare);
35 
36         for(int i=0;i<n;i++)
37         {
38             if(m>=r[i].f)
39             {
40                 m-=r[i].f;
41                 ans+=r[i].j;
42             
43             }
44             else
45             {
46                 ans+=r[i].w*m;
47                 break;
48             }
49         }
50         printf("%.3lf\n",ans);
51     }
52 }