include <bits/stdc++.h>

using namespace std; long long a[1000001], b[1000001], c[1000001]; long long ans = 0;

int main() {

long long n, d, sum = 0;
cin >> n >> d;
for (long long i = 1; i < n; i++) {
	long long x;
	cin >> x;
	a[i] = a[i - 1] + x;
}
for (long long i = 1; i <= n; i++) {
	cin >> b[i];
}
c[1] = b[1];
for (long long i = 2; i <= n; i++)
	c[i] = min(b[i], c[i - 1]);
long long v = 0;
for (long long i = 1; i <= n; i++) {
	if (a[i] <= v)
		continue;
	long long k = ceil(double(a[i] - v ) / d);
	ans += k * c[i];
	v += k * d;
}
cout << ans;

}