/*
TASK:socks
LANG:C++
*/
#include<iostream> //Lybomir Ivanov C10
#include<stdio.h>
using namespace std;
int n,k,i,j,a[30];
double edcena,ceni[30],sum,minc;
int main()
{
scanf("%d%d",&n,&k);
scanf("%d",&a[i]);
for(i=1;i<n;i++)
  {
   scanf("%d",&a[i]);
   a[i]*=a[i-1];
  }
scanf("%lf",&edcena);
for(i=0;i<n;i++)
  scanf("%lf",&ceni[i]);
for(i=n-1;i>-1;i--)
  {
   if((k%a[i])==k) continue;
   for(j=0;j<k/a[i];j++)
     minc=minc + ceni[i];
   k%=a[i];
   break;
  }
sum=minc;
double p=sum;
minc+=ceni[i];
for(i=n-2;i>-1;i--)
  {
   sum=p;
   sum+=((double)(k/a[i]+1))*ceni[i];
   if(sum<minc) minc=sum;
  }
sum=p;
sum+=(double)k*edcena;
if(minc>sum) minc=sum;
printf("%.2lf",minc);
return 0;
}
