#include <bits/stdc++.h>
using namespace std;
int n, m, d[1002], s[1002], t, f, sum, k;
int main() {
  cin >> m >> k;
  for (int i = 2; i <= m + 1; i++) {
    cin >> d[i];
  }
  for (int i = 1; i <= m; i++) {
    cin >> s[i];
  }
  for (int i = 1; i <= m; i++) {
    f += s[i];
    t = max(t, s[i]);
    while (f < d[i + 1]) {
      sum += k, f += t;
    }
    sum += d[i + 1], f -= d[i + 1];
  }
  cout << sum;
}
