1 #include <stdio.h>
 2 #include <algorithm>
 3 
 4 struct treasure {
 5     int p, m;
 6     bool operator<(const treasure&x) const {
 7         return p>x.p;
 8     }
 9 }arr[101];
10 
11 #define p(i) arr[i].p
12 #define m(i) arr[i].m
13 int main()
14 {
15     int v, n;
16     while(~scanf("%d%d", &v, &n)&&v) {
17         for(int i=0; i<n; i++)
18             scanf("%d%d", &p(i), &m(i));
19         std::sort(arr, arr+n);
20         int res=0;
21         for(int i=0; i<n&&v; i++) {
22             if(v<m(i)) {
23                 res+=p(i)*v;
24                 break;
25             }else {
26                 res+=p(i)*m(i);
27                 v-=m(i);
28             }
29         }
30         printf("%d\n", res);
31     }
32     return 0;
33 }