#include<stdio.h>
#include<algorithm>
using namespace std;
struct note {
int uprice;//unite单价
int bulk;//体积
} s[102];
bool judge(note a,note b) {
return a.uprice>b.uprice;
}
int main() {
int n,m;
while(scanf("%d %d",&n,&m),n) {
int sum=0,t;
for(int l=0; l<m; l++) {
scanf("%d %d",&s[l].uprice,&s[l].bulk);
}
sort(s,s+m,judge);
for(int l=0; l<m; l++) {
if(n>s[l].bulk) {
sum=sum+s[l].bulk*s[l].uprice;
n=n-s[l].bulk;
} else {
sum=sum+s[l].uprice*n;
break;
}
}
printf("%d\n",sum);
}
return 0;
}