#include <bits/stdc++.h>

using namespace std;

long long s_min[100010],v[100010],a[100010];
long long n,d,all;


int main(){
	cin >> n >> d;
	for(int i = 1; i < n; i++){
		cin >> v[i];
		all += v[i];
	}
	all++;
	s_min[0] = 1e9;
	for(int i = 1; i <= n; i++){
		cin >> a[i];
		s_min[i] = min(s_min[i-1],a[i]);
	}
	long long now = 1;
	long long mon = 0;
	while(!(now == n)){
		double tmp = a[now]/1.000000/d;
		int tmp3 = tmp;
		if(tmp3 < tmp){//勾石的向上取整 
			int tmp2 = tmp;
			tmp = tmp2+1;
		}
		mon += s_min[now] * tmp;
		now++;
	}
	cout << mon << endl;
	return 0;
}