#include<bits/stdc++.h>
using namespace std;
long long n,d,v[100100],a[100100],ans;
int main(){
	cin>>n>>d;
	for(int i=1;i<n;i++)cin>>v[i];
	for(int i=1;i<=n;i++)cin>>a[i];
	ans+=v[1]*a[1]/d;
	for(int i=1;i<=n;i++){
		if(a[i]<=a[i+1]){
			a[i+1]=a[i];
			ans+=v[i+1]*a[i+1]/d;
		}
		else if(a[i]>a[i+1]){
			ans+=v[i+1]*a[i+1]/d;
		}
	}
	cout<<ans;
	return 0;
}