#include <bits/stdc++.h>
using namespace std;
int main() {
  ios_base ::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);
  long long n, m, i, j, k, l, ans = 0, sum = 0, x, y, z, mx = 0, mn = 2e9, a,
                              flag = 0, b, f;
  cin >> a >> b >> f >> k;
  if (k == 1) {
    x = max(f, (a - f));
    if (x > b) {
      cout << "-1";
      return 0;
    }
  } else if (k == 2) {
    x = max(f, 2 * (a - f));
    if (x > b) {
      cout << "-1";
      return 0;
    }
  } else {
    x = max(2 * f, 2 * (a - f));
    if (x > b) {
      cout << "-1";
      return 0;
    }
  }
  x = b - f;
  y = 0;
  ans = 0;
  while (1) {
    if (y + 1 == k) {
      if (x < (a - f)) {
        ans++;
      }
      cout << ans;
      return 0;
    } else {
      if (x >= 2 * (a - f)) {
        x = x - 2 * (a - f);
      } else {
        ans++;
        x = b - 2 * (a - f);
      }
    }
    y++;
    if (y + 1 == k) {
      if (x < f) {
        ans++;
      }
      cout << ans;
      return 0;
    } else {
      if (x >= 2 * f) {
        x = x - 2 * f;
      } else {
        ans++;
        x = b - 2 * f;
      }
    }
    y++;
  }
  cout << ans;
  return 0;
}
