#include <stdio.h>

int main()
{
    int n,m,a,b;
    while(scanf("%d%d",&n,&m)!=EOF){
        int min_=0;
        for(int i=0; i<m; i+=1){
            int t=n, tmp=0;
            scanf("%d%d",&a,&b);
            if(i==0){
                while(t>=a){
                    t-=a;
                    min_+=b;
                }
                if(t>0) min_+=b;
            }
            else{
                while(t>=a){
                    t-=a;
                    tmp+=b;
                }
                if(t>0) tmp+=b;

                if(tmp<min_) min_=tmp;
            }
        }
        printf("%d\n",min_);
    }


    return 0;
}