```#include<bits/stdc++.h>
using namespace std ;
const long long modd=1e9+7L; 
const int maxn=1e5+10;
typedef pair<long,long>P;
long long n,d,v[maxn],a[maxn],minn=1e18L,ans,you,yu,dis,cha;
int main()
{
	freopen("road.in","r",stdin);
	freopen("road.out","w",stdout); 
	cin>>n>>d;
	for(int i=1;i<n;i++)
	cin>>v[i];
	for(int i=1;i<=n;i++)
	cin>>a[i];
	for(int i=1;i<n;i++)
	{
		minn=min(minn,a[i]);dis-=v[i-1];
		cha=v[i]-dis;
		if(cha<0)continue;
		you=cha/d;yu=cha%d;
		if(yu!=0)you++;
		ans+=you*minn;dis+=you*d;
	}
	cout<<ans;
	return 0;

}