include <bits/stdc++.h>

using namespace std; long long mi[100005], v[100005], a[100005];

int main() {

int n, d;
cin >> n >> d;
v[1] = 0;
for (int i = 2; i <= n; i++) {
	int b;
	cin >> b;
	v[i] = v[i - 1] + b;
}
for (int i = 1; i <= n; i++) {
	cin >> a[i];
}
mi[1] = a[1];
for (int i = 2; i <= n; i++) {
	mi[i] = min(mi[i - 1], a[i]);
}
long long now = 0, ans = 0;
for (int i = 2; i <= n; i++) {
	if (now >= v[i])
		continue;
	int k = ceil((v[i] - now) * 1.0 / d * 1.0);
	ans += mi[i - 1] * k;
	now += k * d;
}
cout << ans;
return 0;

}