#include <bits/stdc++.h>
using namespace std;
bool fl = 0;
int makscap;
int res = 0;
int curr = -1;
int a, cap, stat, ile;
inline void foo() { fl = true; }
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(nullptr);
  cin >> a >> cap >> stat >> ile;
  int st = 0;
  makscap = cap;
  cap += stat;
  while (curr != ile) {
    if (st == 0) {
      ++curr;
      cap -= stat;
      if (cap < 0) foo();
    }
    if (st == 1) {
      cap -= stat;
      if (cap < 0) foo();
      if ((curr == ile - 1 and cap < (a - stat)) or
          (curr < ile - 1 and cap < (a - stat) * 2)) {
        cap = makscap;
        ++res;
      }
    }
    if (st == 2) {
      ++curr;
      cap -= a - stat;
      if (cap < 0) foo();
    }
    if (st == 3) {
      cap -= a - stat;
      if (cap < 0) foo();
      if ((curr == ile - 1 and cap < stat) or
          (curr < ile - 1 and cap < stat * 2)) {
        cap = makscap;
        ++res;
      }
    }
    ++st;
    st %= 4;
  }
  cout << (fl ? -1 : res) << '\n';
}
