Add advert to the_content

<?php
$required_paragraphs = 3;
$content = apply_filters( 'the_content', $post->post_content );
if( substr_count ( $content, '<p>' ) > $required_paragraphs ){
	$contents = explode( "</p>", $content );
	$paragraphs = 1;
	foreach( $contents as $content ){
		echo $content;
		if( $paragraphs == $required_paragraphs ){
			/* Advert code here */
		}
		echo "</p>";
		$paragraphs++;
	}
}
?>