#include <bits/stdc++.h>
using namespace std;
int a, b, f, k;
bool ok(int total) {
  int cur = b, dir = 0;
  int times = 0;
  if (b < f || b < (a - f)) return 0;
  for (int i = 0; i < k; i++) {
    if (dir == 0) {
      if (cur >= a) {
        if (i == k - 1) return times <= total;
        if (cur - a >= a - f)
          cur -= a;
        else {
          times++;
          cur = b - (a - f);
        }
      } else {
        if (cur < f)
          return 0;
        else {
          times++;
          cur = b - (a - f);
        }
      }
      dir = 1;
    } else {
      if (cur >= a) {
        if (i == k - 1) return times <= total;
        if (cur - a >= f)
          cur -= a;
        else {
          times++;
          cur = b - f;
        }
      } else {
        if (cur < (a - f))
          return 0;
        else {
          times++;
          cur = b - f;
        }
      }
      dir = 0;
    }
  }
  return times <= total;
}
int main() {
  cin >> a >> b >> f >> k;
  int l = 0, r = k;
  int ans = -1;
  while (l <= r) {
    int mid = (l + r) >> 1;
    if (ok(mid)) {
      ans = mid;
      r = mid - 1;
    } else {
      l = mid + 1;
    }
  }
  cout << ans << endl;
  return 0;
}
