#include <bits/stdc++.h>
using namespace std;
long long a[10086],aa[100008],minn = 111111111111,sum = 0,lon = 0,yu = 0;
int main(){
	int n,d;
	cin>>n>>d;
	for(int i = 1;i<n;i++){
		cin>>a[i];
	}
	for(int i = 1;i<n;i++){
		cin>>aa[i];
		minn = min(minn,aa[i]);
		if(sum>=a[i]){
			continue;
		}
		else{
       		lon += ceil(a[i]*1.0/d)*minn;
       		sum += ceil(a[i]*1.0/d)*d;
       		sum -= a[i];			
		}

	}
	cin>>aa[n];
	cout<<lon;
	return 0;
}