#include<cstdio>

 #include<cstring>

 #include<iostream>

 #include<algorithm>

 #include<string>

 #include<cmath>

 #include<set>

 #include<queue>

 #include<map>

 #include<stack>

 #include<vector>

 #include<list>

 #include<deque>

 #include<sstream>

 #include<cctype>

 #define REP(i, n) for(int i = 0; i < (n); i++)

 #define FOR(i, s, t) for(int i = (s); i < (t); i++)

 #define MEM(a, x) memset(a, x, sizeof(a));

 #define DEBUG(x) cout<<x<<endl;

 #define DBG cout<<"----------------"<<endl;

 #define mid(x, y) x + (y - x)/ 2

 #define lc o<<1

 #define rc o<<1|1

 using namespace std;

 typedef int64_t INT;

 typedef long long ll;

 typedef unsigned long long ull;

 typedef pair<int, int> Pair;

 const int maxn = 1e3 + ;

 const double eps = 1e-;

 const int INF =  << ;

 const int dir[][] = {,,,,,-,-,};

 const double pi = 3.1415926535898;

 int T, n, m, cases;

 struct node

 {

     double j, f;

     bool operator < (const node& a)const

     {

         return (j / f) > (a.j / a.f);

     }

 };

 node a[maxn];

 int main()

 {

     while(cin >> m >> n)

     {

         if(m == - && n == -)break;

         memset(a, , sizeof(a));

         for(int i = ; i < n; i++)

         {

             cin >> a[i].j >> a[i].f;

         }

         sort(a, a + n);

         double ans = ;

         for(int i = ; i < n; i++)

         {

             if(m >= a[i].f)

             {

                 ans += a[i].j;

                 m -= a[i].f;

             }

             else

             {

                 ans += (m * a[i].j / a[i].f);

                 break;

             }

         }

         printf("%.3f\n", ans);

     }

     return ;

 }