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

int main() {
    int n, d;
    cin >> n >> d;
    vector<int> v(n - 1), a(n);
    for (int i = 0; i < n - 1; i++) {
        cin >> v[i];
    }
    for (int i = 0; i < n; i++) {
        cin >> a[i];
    }

    int min_cost = 0;
    int current_fuel = 0;
    for (int i = 0; i < n - 1; i++) {
        if (current_fuel < v[i]) {
            min_cost += a[i + 1] * (v[i] - current_fuel) / d;
            current_fuel = v[i];
        }
        current_fuel -= v[i];
    }

    cout << min_cost << endl;
    return 0;
}