I'm trying to add an rss mod to my forum but i only want certain forums to display. The code to do this in the rss.php file is as follows:

// Which forum do you want posts from (forum_id)? specified in the url with "f=". Defaults to all (public) forums.
$forum_id = ( isset($HTTP_GET_VARS['f']) ) ? intval($HTTP_GET_VARS['f']) : '';
$sql_forum_where = ( !empty($forum_id) ) ? ' AND f.forum_id = ' . $forum_id : ' ';

What i dont understand is which bit do i put the forum numbers in?
and
do i have to put f=# in everytime or just the number itself?

Any help would be appreciated - thanks.