#include<bits/stdc++.h>
using namespace std;
int n,d,v[100005],a[100005],sum,ans=100000,g,y,yl;
int main(){
	cin>>n>>d;
	for(int i=1;i<=n-1;i++)
	{
		cin>>v[i];
	}
	for(int i=1;i<=n;i++)
	{
		cin>>a[i];
	}
	for(int i=1;i<=n-1;i++)
	{

		ans=min(ans,a[i]);
		y=g;
		g=0;
		yl=(v[i]-y+d-1)/d;
		g+=yl*d;
		g-=v[i];
		sum+=yl*ans;
	 } 
	 cout<<sum;
	return 0;
}