function insert_ad_block( $text ) {

    if ( is_single() ) :

        $ads_text = '<div class="wpselect_middle_content">-----reklam-----</div>';
        $split_by = "\n";
        $insert_after = 5; //number of paragraphs

        // make array of paragraphs
        $paragraphs = explode( $split_by, $text);

        // if array elements are less than $insert_after set the insert point at the end
        $len = count( $paragraphs );
        if (  $len < $insert_after ) $insert_after = $len;

        // insert $ads_text into the array at the specified point
        array_splice( $paragraphs, $insert_after, 0, $ads_text );

        // loop through array and build string for output
        foreach( $paragraphs as $paragraph ) {
            $new_text .= $paragraph; 
        }

        return $new_text;

    endif;

    return $text;

}
add_filter('the_content', 'insert_ad_block');