Set expressions

[#1]

Table A
S.no	code	Doc.code	Total
1	ILE	1001	20
2	ILE	1002	40
3	ILE	1003	30
4	ILE	1004	50
5	ILE	1005	80
Table B
s.no	code	Ref code	Ref Doc code	Total
1	SRLE	ILE	1002	20
2	SRLE	ILE	1004	40
3	SRLE	ILE	1005	80

hI above are my two tables .

table A is invoice  and table B is sales return.

i need to find the sum of sales by subtracting the second table . as if sales return has been done it wont be consider as profit.

how can i achieve this in expression. please help

Thanks

Vignesh