#include <bits/stdc++.h>
using namespace std;
template <class T>
void show(T *a, int n) {
  for (int i = 0; i < n; i++) cout << a[i] << "  ";
  puts("");
}
template <class T>
void queue_clear(T *a, int n) {
  for (int i = 0; i < n; i++) {
    while (!a->empty()) a->pop();
  }
}
template <class T>
void Bit_add(T *a, int x, int p, int MAX) {
  while (p < MAX) {
    a[p] += x;
    p += p & (-p);
  }
}
template <class T>
T Bit_sum(T *a, int p) {
  T ret = 0;
  while (p) {
    ret += a[p];
    p -= p & (-p);
  }
  return ret;
}
const int MAXN = 1007;
int d[MAXN], s[MAXN];
int main() {
  int m, k;
  while (~scanf("%d%d", &m, &k)) {
    int i;
    for (i = 1; i <= m; i++) scanf("%d", d + i);
    for (i = 1; i <= m; i++) scanf("%d", s + i);
    int tank = s[1], mx = s[1];
    int p = 1, ans = 0;
    while (p <= m) {
      if (tank < d[p]) {
        int t = ((d[p] - tank) + mx - 1) / mx;
        ans += t * k;
        tank += t * mx;
      }
      tank -= d[p];
      ans += d[p];
      p++;
      mx = max(mx, s[p]);
      tank += s[p];
    }
    printf("%d\n", ans);
  }
  return 0;
}
