P9749

#include<bits/stdc++.h>
using namespace std;
long long n, d, a[100005], v[100005], ans, t, s;
int main() {
	scanf("%d%d", &n, &d);
	for (int i = 1; i < n; i++) {
		scanf("%d", &v[i]);
	}
	for (int i = 1; i <= n; i++) {
		scanf("%d", &a[i]);
	}
	t = 1;
	for (int i = 1; i <= n; i++) {

		if (a[i] < a[t]) {
			ans += s / d * a[t];
			s = 0;
			t = i;
		} else if (i == n) {
			ans += s / d * a[t];
		}
		s += v[i];
	}

	printf("%d", ans);
	return 0;
}