#include<bits/stdc++.h>
using namespace std;
int n,d,v[100005];
long long h=0,aa=1e5+10,ans=0,a;
int main(){
	freopen("road.in","r",stdin);
	freopen("road.out","w",stdout);
	cin>>n>>d;
	v[1]=0;
	for(int i=2;i<=n;i++) cin>>v[i];
	for(int i=1;i<=n;i++){
		cin>>a;
		long long b=ceil((v[i]-h)*1.0/d);
		ans=ans+b*aa;
		h=b*d-v[i];
		aa=min(a,aa);
	}
	cout<<ans;
	return 0;
}