1 #include<cstdio>
 2 #include<cstring>
 3 #include<algorithm>
 4 using namespace std;
 5 const int maxn=1010;
 6 struct javabean{
 7     int j,f;
 8     double v;
 9 }value[maxn];
10 bool cmp(javabean a,javabean b){
11     return a.v>b.v;
12 }
13 int main()
14 {
15     int m,n;
16     while(scanf("%d%d",&m,&n)){
17         if(m==-1&&n==-1) break;
18         for(int i=0;i<n;i++){
19             scanf("%d%d",&value[i].j,&value[i].f);
20             value[i].v=(double)value[i].j/(value[i].f*1.0);
21         }
22         sort(value,value+n,cmp);
23         double sum=0.0;
24         for(int i=0;i<n;i++){
25             if(m>=value[i].f){
26                 sum+=value[i].j;
27                 m-=value[i].f;
28             }
29             else{
30                 sum+=value[i].v*m;
31                 break;
32             }
33         }
34         printf("%.3f\n",sum);
35     }
36     return 0;
37 }