rt, 0pts

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

const int N = 1e5 + 10;
long long s[N];
int a[N];

int main() {
  int n, d;
  cin >> n >> d;
  for (int i = 2; i <= n; i++) {
    long long tmp;
    cin >> tmp;
    s[i] = s[i - 1] + tmp;
  }
  for (int i = 1; i <= n; i++) cin >> a[i];
  vector<int> p;
  p.push_back(1);
  int minP = a[1];
  for (int i = 2; i < n; i++) {
    if (a[i] < minP) {
      minP = a[i];
      p.push_back(i);
    }
  }
  p.push_back(n);
  long long ans = 0;
  for (int i = 0; i < p.size() - 1; i++) ans += a[p[i]] * ((s[p[i + 1]] - s[p[i]]) / d + 0.5);
  cout << ans;
  return 0;
}