add_action( 'genesis_after_entry', 'sk_ad' );
/**
 * Insert one random ad after every second post.
 *
 * Scope: Posts page and all category archives.
 *
 * @author Sridhar Katakam
 * @link   http://sridharkatakam.com/display-random-ad-every-second-post-genesis/
 */
function sk_ad() {

	// if we are not on Posts page or category archive, abort.
	if ( ! ( is_home() || is_category() ) ) {
		return;
	}

	// if Ads by datafeedr.com plugin is not active, abort.
	if ( ! function_exists( 'dfads' ) ) {
		return;
	}

	// if this is a paginated page, abort.
	if ( is_paged() ) {
		return;
	}

	global $wp_query;

	if( $wp_query->current_post == 1 || $wp_query->current_post == 3 ) { // if 2nd or 4th post
		echo dfads( 'groups=-1&limit=1&orderby=random' );
	}

}