#include <bits/stdc++.h>
using namespace std;
int main() {
  long long a, b, f, k;
  cin >> a >> b >> f >> k;
  if (f > b) {
    cout << "-1";
    return 0;
  }
  if (k == 1) {
    if (b >= a) {
      cout << "0";
      return 0;
    }
    if (f > b || (a - f) > b) {
      cout << "-1";
      return 0;
    } else {
      cout << "1";
      return 0;
    }
  }
  int count = 0, cap = 0;
  if (k > 2) {
    if (b < 2 * (a - f) || (2 * f) > b) {
      cout << "-1";
      return 0;
    }
  }
  if (k == 2) {
    if (b < 2 * (a - f) || (f) > b) {
      cout << "-1";
      return 0;
    }
  }
  cap = b - f;
  int j = 0;
  while (k > 1) {
    if (j % 2 == 0) {
      if (cap < 2 * (a - f)) {
        count++;
        cap = b;
      }
      cap -= 2 * (a - f);
      j++;
    } else {
      if (cap < 2 * f) {
        count++;
        cap = b;
      }
      cap -= 2 * f;
      j++;
    }
    k--;
  }
  if (j % 2) {
    if (cap < f) count++;
  } else {
    if (cap < (a - f)) count++;
  }
  cout << count;
  return 0;
}
