#include <bits/stdc++.h>
using namespace std;
long long n, m, res, t, k, o;
long long a[1000100], b[1000100];
priority_queue<int, vector<int>, greater<int> > q;
int main() {
  scanf("%lld %lld", &n, &k);
  for (long long i = 1; i <= n; i++) {
    scanf("%lld", &a[i]);
  }
  scanf("%lld", &m);
  for (long long i = 1; i <= n; i++) {
    scanf("%lld", &b[i]);
  }
  res = 0;
  for (int i = 1; i <= n; i++) {
    q.push(b[i]);
    while (k < a[i] && !q.empty()) {
      k = k + m;
      res += q.top();
      q.pop();
    }
    if (k < a[i]) {
      res = -1;
      break;
    }
  }
  printf("%lld\n", res);
}
