#include <iostream>
#include <climits>
#include <cmath>
const int N = 1e5 + 50;
int main() {
	int n, d;
	std::cin >> n >> d;
	int v[N] = {};
	for (int i = 2; i <= n; i++) {
		std::cin >> v[i];
	}
	int a[N] = {}, Min[N] = {INT_MAX};
	for (int i = 1; i <= n; i++) {
		Min[N] = INT_MAX;
		std::cin >> a[i];
		Min[i] = std::min(Min[i - 1], a[i]);
	}
	int V = 0, money = 0;
	for (int i = 2; i <= n; i++) {
		if (V < v[i]) {
			while (V < v[i]) {
				money += Min[i - 1];
				V += d;
			}
			V -= v[i];
		}
	}
	std::cout << money;
	return 0;
}