1 #include <iostream>
 2 #include <vector>
 3 #include <algorithm>
 4 using namespace std;
 5 struct WareHouse
 6 { 
 7     int JB,CF;
 8     double ratio;
 9 };
10 bool cmp(WareHouse a,WareHouse b)
11 {
12     return a.ratio  >  b.ratio;
13 }  
14 int main()
15 {
16     int m,n;
17     while(scanf_s("%d%d",&m,&n)&&m!=-1&&n!=-1)
18     {
19         double answer=0;
20         //int*p = new int[n];
21         vector<WareHouse> WH;
22         int i=n;
23         while(i--)
24         {
25             WareHouse temp;
26             scanf_s("%d%d",&temp.JB,&temp.CF);
27             temp.ratio = (double)temp.JB/temp.CF;
28             WH.push_back(temp);
29         }
30     //    sort(WH.begin(),WH.end(),[](const WareHouse &a,const WareHouse&b ){ return a.ratio>b.ratio;});
31         sort(WH.begin(),WH.end(),cmp);
32         for (int j = 0; j < n; j++)
33         {
34             if(m<=WH[j].CF)
35             {
36                 answer=answer+m*WH[j].ratio;
37                 m=0;
38             }
39             else
40             {
41                 answer=answer+WH[j].JB;
42                 m-=WH[j].CF;
43             }
44         }
45         printf("%.3f\n",answer);
46     }
47     return 0;
48 }