I made a loop but it doesn't work.

here's what I did:

<?php if(is_tree('4')) { ?>
<?php
$show_after_p = 2;
$content = apply_filters('the_content', $post->post_content);
if(substr_count($content, '<p>') > $show_after_p)
{
    $contents = explode("</p>", $content);
    $p_count = 1;
    foreach($contents as $content)
    {
        echo $content;

        if($p_count == $show_after_p)
        {
        ?>
                YOUR AD CODE GOES HERE
        < ?
        }
        echo "</p>";
        $p_count++;
    }
}
?>
<?php else : ?>
        <?php the_content('<p class="serif">Read the rest of this page &raquo;</p>'); ?>  
         <?php endif; } ?>

I need to make it work but don't know how. I'm guessing it's a simple syntax error I'm not seeing?