#include <bits/stdc++.h>
using namespace std;
int main(){
freopen("road.in","r",stdin);
freopen("road.out","w",stdout);
long long n,d,v[100010],a[100010],sum=0,pre=0,mn=1000001;
cin>>n>>d;
for(int i=2;i<=n;i++){cin>>v[i];}
for(int i=1;i<=n;i++){
cin>>a[i];
if(i>1){
if(pre>=v[i]){
pre-=v[i];
continue;
}
mn=min(mn,a[i-1]);
int w=(v[i]-pre+d-1)/d;
sum+=mnw;
pre+=wd-v[i];
}
}
cout<<sum;
fclose(stdin);
fclose(stdout);
return 0;
}