#include <bits/stdc++.h>
int main() {
  int i, a, b, f, k, ans = 0, cur, need;
  scanf("%i%i%i%i", &a, &b, &f, &k);
  cur = b - f;
  need = 2 * (a - f);
  for (i = 1; i < k; i++) {
    if (cur < 0) {
      ans = -1;
      break;
    }
    if (cur < need) {
      cur = b;
      ans++;
    }
    cur -= need;
    if (need == 2 * f)
      need = 2 * (a - f);
    else
      need = 2 * f;
  }
  need /= 2;
  if (cur < 0)
    ans = -1;
  else if (cur < need) {
    cur = b;
    cur -= need;
    if (cur < 0)
      ans = -1;
    else
      ans++;
  }
  printf("%i", ans);
  return 0;
}
