1 #include <iostream>
 2 #include <algorithm>
 3 #include<iomanip>
 4 using namespace std;
 5 #define maxn 1005
 6 struct Room
 7 {
 8     int f,j;
 9     double value;
10 }room[maxn];
11 int cmp(Room a,Room b)
12 {
13     return a.value>b.value;
14 }
15 int main()
16 {
17     int m,n;
18     while(cin>>m>>n)
19     {
20         if(m==-1&&n==-1) break;
21         for(int i=0;i<n;i++)
22         {
23          cin>>room[i].j>>room[i].f;
24          room[i].value=(1.0*room[i].j) /room[i].f;
25         }
26         sort(room,room+n,cmp);//排序
27         double total=0;
28           for(int i=0;i<n;i++)
29           {
30               if(m<=room[i].f){total+=(room[i].j*1.0*m)/room[i].f;break;}
31               else {total+=room[i].j;m-=room[i].f;}
32           }
33         cout<<setiosflags(ios::fixed)<<setprecision(3)<<total<<endl;
34 
35     }
36     return 0;
37 }