rt 民间20pts->CCF0pts

#include<bits/stdc++.h>
using namespace std;
int n,d,a[100010],v[100010];
int main(){
    freopen("road.in","r",stdin);
    freopen("road.out","w",stdout);
    long long len=0;
    cin>>n>>d;
    int i,j;
    for(i=1;i<n;i++){
        cin>>v[i];
        len+=v[i];
    }
    for(i=1;i<=n;i++)cin>>a[i];
    cout<<ceil(1.0*len/d)*a[1]<<endl;
    return 0;
}