#include <bits/stdc++.h>
using namespace std;
const int N = 1e9 + 7;
void solve() {
  long long n, curr, val, ans = 0;
  cin >> n >> curr;
  vector<long long> a(n), cost(n);
  for (long long i = 0; i < n; i++) cin >> a[i];
  cin >> val;
  for (long long i = 0; i < n; i++) cin >> cost[i];
  multiset<long long> s;
  for (long long i = 0; i < n; i++) {
    s.insert(cost[i]);
    if (curr < a[i]) {
      long long x = (a[i] - curr + val - 1) / val;
      if (x > s.size()) {
        ans = -1;
        break;
      }
      while (x--) {
        ans += *s.begin();
        curr += val;
        s.erase(s.begin());
      }
    }
  }
  cout << ans;
  cout << "\n";
}
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);
  int t = 1;
  while (t--) {
    solve();
  }
  return 0;
}
