#include <bits/stdc++.h>
using namespace std;
int main() {
  int m, k, d[2000], s[2000], n = 0, big = 0, ans = 0;
  cin >> m >> k;
  for (int i = 0; i < m; i++) cin >> d[i];
  for (int i = 0; i < m; i++) cin >> s[i];
  for (int i = 0; i < m; i++) {
    n += s[i];
    if (s[i] > big) big = s[i];
    while (n < d[i]) {
      n += big;
      ans += k;
    }
    n -= d[i];
    ans += d[i];
  }
  cout << ans << endl;
}
