#include <bits/stdc++.h>
using namespace std;
const int OO = 1e9;
const double EPS = 1e-9;
template <class T>
void _db(const char* dbStr, T e) {
  cout << dbStr << " = " << e << endl;
}
template <class T, class... L>
void _db(const char* dbStr, T e, L... r) {
  while (*dbStr != ',') cout << *dbStr++;
  cout << " = " << e << ',';
  _db(dbStr + 1, r...);
}
template <class S, class T>
ostream& operator<<(ostream& o, const map<S, T>& v) {
  o << "[";
  int i = 0;
  for (const pair<S, T>& pr : v)
    o << (!i++ ? "" : ", ") << "{" << pr.first << " : " << pr.second << "}";
  return o << "]";
}
template <template <class, class...> class S, class T, class... L>
ostream& operator<<(ostream& o, const S<T, L...>& v) {
  o << "[";
  int i = 0;
  for (const auto& e : v) o << (!i++ ? "" : ", ") << e;
  return o << "]";
}
template <class S, class T>
ostream& operator<<(ostream& o, const pair<S, T>& pr) {
  return o << "(" << pr.first << ", " << pr.second << ")";
}
ostream& operator<<(ostream& o, const string& s) {
  for (const char& c : s) o << c;
  return o;
}
template <class T>
using V = vector<T>;
template <class T>
using VV = V<V<T>>;
template <class T>
using VVV = VV<V<T>>;
using ll = long long;
using pii = pair<int, int>;
using vi = V<int>;
using vii = V<pii>;
using vvi = VV<int>;
using mii = map<int, int>;
using umii = unordered_map<int, int>;
using si = set<int>;
using usi = unordered_set<int>;
int N, K, A;
vi X, C;
int main() {
  ios::sync_with_stdio(false);
  cout.precision(10);
  cin.tie(0);
  cin >> N >> K;
  X.assign(N + 1, 0);
  for (int i = 1; i <= N; ++i) {
    cin >> X[i];
  }
  cin >> A;
  C.assign(N + 1, 0);
  for (int i = 1; i <= N; ++i) {
    cin >> C[i];
  }
  multiset<int> s;
  ll ans = 0;
  for (int i = 1; i <= N; ++i) {
    s.insert(C[i]);
    while (K < X[i]) {
      if (s.empty()) {
        cout << "-1\n";
        return 0;
      }
      ans += *s.begin();
      s.erase(s.begin());
      K += A;
    }
  }
  cout << ans << '\n';
  return 0;
}
