#include <bits/stdc++.h>
using namespace std;
const long long N = 2e5 + 7;
long long power(long long x, long long y) {
  long long res = 1;
  x = x;
  while (y > 0) {
    if (y & 1) res = (res * x);
    y = y >> 1;
    x = (x * x);
  }
  return res;
}
template <typename T>
T gcd(T a, T b) {
  if (a == 0) return b;
  return gcd(b % a, a);
}
template <typename T>
T powm(T a, T b, long long m) {
  T cnt = 1;
  while (b > 0) {
    if (b % 2 == 1) cnt = (cnt * a) % m;
    b /= 2;
    a = (a * a) % m;
  }
  return cnt % m;
}
long long ncr(long long n, long long r) {
  long long res = 1;
  if (r > n) return 0;
  if (r > n - r) r = n - r;
  for (long long i = 0; i < r; i++) {
    res *= (n - i);
    res /= (i + 1);
  }
  return res;
}
long long n, k, x[N], r, c[N], cur, ans, t;
void solve() {
  cin >> n >> k;
  cur = k;
  for (long long int i = (1); i <= (n); ++i) cin >> x[i];
  cin >> r;
  for (long long int i = (1); i <= (n); ++i) cin >> c[i];
  priority_queue<long long, vector<long long>, greater<long long> > pq;
  for (long long int i = (1); i <= (n); ++i) {
    pq.push(c[i]);
    if (cur >= x[i]) continue;
    if (x[i] > cur) {
      while (!pq.empty()) {
        t = pq.top();
        pq.pop();
        cur += r;
        ans += t;
        if (cur >= x[i]) break;
      }
    }
    if (cur < x[i]) {
      cout << "-1\n";
      return;
    }
  }
  cout << ans << "\n";
}
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(0);
  cout.tie(0);
  int t;
  t = 1;
  while (t--) solve();
  return 0;
}
