#include <bits/stdc++.h>
using namespace std;
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  int a, b, f, k;
  cin >> a >> b >> f >> k;
  int cur_pos = f;
  int cur_fuel = b - f;
  int goal = a;
  int ans = 0;
  while (k > 0) {
    if (cur_fuel < 0) {
      cout << -1 << '\n';
      return 0;
    } else if (k == 1) {
      if (abs(cur_pos - goal) > b) {
        cout << -1 << '\n';
        return 0;
      } else if (abs(cur_pos - goal) > cur_fuel) {
        cur_fuel = b - abs(cur_pos - goal);
        ans++;
      } else {
        cur_fuel = cur_fuel - abs(cur_pos - goal);
      }
      k--;
    } else {
      if (2 * abs(cur_pos - goal) > b) {
        cout << -1 << '\n';
        return 0;
      } else if (2 * abs(cur_pos - goal) > cur_fuel) {
        cur_fuel = b - 2 * abs(cur_pos - goal);
        ans++;
      } else {
        cur_fuel = cur_fuel - 2 * abs(cur_pos - goal);
      }
      k--;
      goal = goal == 0 ? a : 0;
    }
  }
  cout << ans << '\n';
  return 0;
}
