Insert ads in single post content

add_filter( 'the_content', 'prefix_t4b_post_ads' );
function prefix_t4b_post_ads( $content ) {
    $ad_code = '<div id="t4b-post-ads">
            <!-- Ad Start -->
            // Paste here your Ads code
            <!-- Ad End -->
            <div class="post_ad_label">Advertise</div>
            </div>';
    if ( is_single() && ! is_admin() ) {
        return prefix_t4b_after_paragraph( $ad_code, $content );
    }
    return $content;
}
function prefix_t4b_after_paragraph( $insertion, $content ) {
    $paragraphs = $insertion . $content;
    return $paragraphs;
}