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