i am trying to learn something here ok not copying or doing any cheating

this is by far i have tired

;with cte (month, totalchargedamount)
As
(
Select  c.monthindex ,sum(a.charged_amount)as totalcharged
From lineitems a  join invoice b on a. invoiceID = b. invoiceID
right Join months c on b.month = c.monthIndex
group by monthindex)
,cta(month,total)
as
(

select b.month,(a.totalchargedamount- b.totalchargedamount)/100 as total
from cte a  join cte b on b.month-1 = a.month)

select a.month,totalchargedamount,total from cte a left  join cta b
on a.month = b.month
but  i want to learn good easy way than this