#include <bits/stdc++.h>
using namespace std;
int main() {
  long long a, b, f, k;
  cin >> a >> b >> f >> k;
  long long be, af, ans = 0, cur = b;
  be = f;
  be *= 2;
  af = a - f;
  af *= 2;
  for (long long i = 1; i <= k; i++) {
    if (i == 1)
      cur -= f;
    else if (i & 1)
      cur -= be;
    else
      cur -= af;
    if (cur < 0) {
      printf("-1\n");
      return 0;
    }
    if (i == k) {
      if (i & 1) {
        if (cur < a - f) cur = b, ans++;
        if (cur < a - f) {
          printf("-1\n");
          return 0;
        }
      } else {
        if (cur < f) cur = b, ans++;
        if (cur < f) {
          printf("-1\n");
          return 0;
        }
      }
    } else {
      if (i & 1) {
        if (cur < af) cur = b, ans++;
      } else if (cur < be)
        cur = b, ans++;
    }
  }
  cout << ans << endl;
  return 0;
}
