gistfile1.php

<?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
		<?php
		}
		echo "</p>";
		$p_count++;
	}
}
?>