<?php

$field = get_field('related-post');
if (preg_match("/page_id=(.+)/", $field, $matches)) {
  $page = get_page($matches[1]);
  echo 'Related page: ' . $page->post_title;
} elseif (preg_match("/p=(.+)/", $field, $matches)) {
  $post = get_post($matches[1]);
  echo 'Related post: ' . $post->post_title;
} else {
  echo 'No related page or post';
}          

?>