Subtract value 2 from value 1 in sql

Question

with cte as
select userid, parentid from user_detail where userid = '100002'
union all select t.userid, t.parentid
from user_detail t inner join cte on cte.userid = t.parentid
SELECT (10 - B.AMOUNT) AS DUE FROM cte AS A
CROSS APPLY (SELECT SUM(Pairs) AS AMOUNT FROM payout WHERE user_id = '100002') AS B
WHERE A.userid = '100002'
i have to change below line from above code :
SELECT (10 - B.AMOUNT) AS DUE FROM cte AS A
i have to subtract, total number of rows in cte from B.Amount instead of 10 - B.Amount.
i.e No.of rows in cte - B.Amount.
when i try something like below it gives error :
SELECT (count(*) - B.AMOUNT) AS DUE FROM cte AS A
Error is : 
Msg 8120, Level 16, State 1, Line 8
Column 'B.AMOUNT' is invalid in the select list because it is not contained in either an aggregate function or the GROUP BY clause.
otherwise it works fine..
Hope you understand my question..

Answer 1

If you want to subtract total rows of CTE then you dont even require the APPLy,COUNT() OVER etc.
you can just simplify it as below
with cte as
select userid, parentid from user_detail where userid = '100002'
union all select t.userid, t.parentid
from user_detail t inner join cte on cte.userid = t.parentid
SELECT ((SELECT count(*) FROM cte) - AMOUNT) AS DUE
FROM payout
WHERE user_id = '100002'
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