list pages even on a subpage, but ancestor too.

this code lists the pages even if on the subpage, but not the ancestor

<?php
  if($post->post_parent)
  $children = wp_list_pages("title_li=&child_of=".$post->post_parent."&echo=0");
  else
  $children = wp_list_pages("title_li=&child_of=".$post->ID."&echo=0");
  if ($children) { ?>
  <ul>
  <?php echo $children; ?>
  </ul>
  <?php } ?>

I need the ancestor to be listed too, not only it`s child pages

thank you