Need Quick Help With A Sql Query

this query works, i want to add a 4th column that is the value of the 3rd column subracted from the value of the 2nd column, how can i add this??

 SELECT `tagid` AS w1, (

SELECT count( `value` )
FROM tags
WHERE `value` =1
AND `tagid` = w1
) AS w2, (

SELECT count( `value` )
FROM tags
WHERE `value` =0
AND `tagid` = w1
) AS w3
FROM tags
WHERE `value` > -1
GROUP BY `tagid`
ORDER BY w2 DESC