#include <stdio.h>
#include <stdlib.h>
typedef struct{
    int p; //单价
    int q; //最多牛奶量
}Hash;
Hash hash[5000];
int cmp(const void *a,const void *b){
    return ((Hash*)a)->p < ((Hash *)b)->p ? -1 : 1;//升序排列
}
int main(){
    int m, n;
    while(scanf("%d %d",&n, &m) != EOF){
        for(int i = 0;i < m;++i)
            scanf("%d %d",&hash[i].p,&hash[i].q);   //读入数据
        qsort(hash,m,sizeof(Hash),cmp);
        int ans = 0;
        for(int i = 0;n > 0;++i){
            int tmp = n;
            if(n > hash[i].q) tmp = hash[i].q;
            ans += tmp * hash[i].p;
            n -= tmp;
        }
        printf("%d",ans);
    }
    return 0;
}