#include <bits/stdc++.h>
int a, b, f, k;
int p, x, ans;
bool calc() {
  x = b;
  p = ans = 0;
  while (k--) {
    if (p == 0) {
      if (x < f) return 0;
      x -= f, p = f;
      if (k && x < (a - f) * 2) x = b, ans++;
      if (!k && x < a - f) x = b, ans++;
      if (x < a - f) return 0;
      p = a, x -= (a - f);
    } else {
      if (x < a - f) return 0;
      x -= (a - f), p = f;
      if (k && x < f * 2) x = b, ans++;
      if (!k && x < f) x = b, ans++;
      if (x < f) return 0;
      p = 0, x -= f;
    }
  }
  return 1;
}
int main() {
  scanf("%d%d%d%d", &a, &b, &f, &k);
  if (calc())
    printf("%d\n", ans);
  else
    printf("%d\n", -1);
  return 0;
}
