#include <bits/stdc++.h>
const int M = 1e9 + 7, N = 2 * 1e5 + 3;
using namespace std;
int main() {
  long long a, b, f, k, b0;
  cin >> a >> b >> f >> k;
  b0 = b;
  int ans = 0;
  if (b < f || b < a - f) {
    cout << -1;
    return 0;
  }
  b0 = b - f;
  if (b0 < 0) {
    cout << -1;
    return 0;
  }
  bool q = 1;
  while (k > 1) {
    if (q) {
      if (b0 < (a - f) * 2) b0 = b, ans++;
      b0 -= (a - f) * 2;
      if (b0 < 0) {
        cout << -1;
        return 0;
      }
      q = 0;
      k--;
    } else {
      if (b0 < 2 * f) b0 = b, ans++;
      b0 -= 2 * f;
      if (b0 < 0) {
        cout << -1;
        return 0;
      }
      q = 1;
      k--;
    }
  }
  if (q && b0 < a - f || !q && b0 < f)
    cout << ans + 1;
  else
    cout << ans;
  return 0;
}
