#include <bits/stdc++.h>
using namespace std;
template <typename G>
inline void read(G &x) {
  x = 0;
  char c;
  int dau = 1;
  while (!isdigit(c = getchar())) {
    if (c == '-') dau = -1;
  };
  do {
    x = x * 10 + c - '0';
  } while (isdigit(c = getchar()));
  x = dau * x;
}
template <typename G>
inline void write(G x) {
  if (x > 9) write(x / 10);
  putchar(x % 10 + '0');
}
template <class T>
inline T min(T a, T b, T c) {
  return min(a, min(b, c));
}
template <class T>
inline T min(T a, T b, T c, T d) {
  return min(a, min(b, c, d));
}
template <class T>
inline T max(T a, T b, T c) {
  return max(a, max(b, c));
}
template <class T>
inline T max(T a, T b, T c, T d) {
  return max(a, max(b, c, d));
}
const int MAXN = 1e5 * 5;
const int base = 1e9 + 7;
const int N = 500;
int n;
int x[MAXN], c[MAXN];
priority_queue<int> h;
long long ans, A, k;
int main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  cin >> n >> k;
  for (int i = (1); i <= (n); ++i) cin >> x[i];
  cin >> A;
  for (int i = (1); i <= (n); ++i) cin >> c[i];
  for (int i = (1); i <= (n); ++i) {
    h.push(-c[i]);
    if (k < x[i]) {
      while (k < x[i] && h.size()) {
        k += A;
        ans -= h.top();
        h.pop();
      }
      if (k < x[i]) {
        cout << -1;
        return 0;
      }
    }
  }
  cout << ans << '\n';
  cerr << "\nTime elapsed: " << 1000 * clock() / CLOCKS_PER_SEC << "ms\n";
  return 0;
}
