#include <bits/stdc++.h>
using namespace std;




long long c[100005] = {0}, d[100005] = {0};

int main() {
	long long a = 0, b = 0, g = 0;
	cin >> a >> b;

	for (long long i = 0; i < a - 1; i++) {

		cin >> c[i];
	}

	for (long long i = 0; i < a; i++) {

		cin >> d[i];
	}

	long long i = 0, last = 0;

	while (i != a - 1) {
		long long j = i + 1, len = 0;
		len += c[i];

		while (j < a - 1 && d[j] >= d[i]) {
			len += c[j];
			j++;
		}

		len -= last;
		long long x = len / b;

		if (len % b != 0) {
			x++;
		}

		g += x * d[i];
		last = x * b - len;
		i = j;
	}

	cout << g;
}