Adding or subtracting

I'm in trouble. I have a table with some fields, one of them is an amount of money.
One field indicates whether the amount is negetive or positive. This fields has only two possible values, 'N' and 'R'. 'N' means the amount is negative and 'R' means the amount is positive.

The problem I have is that I have to sum these amounts to give a total for each customer and report them as

customer1 total amount1
customer2 total amount2

I don't know how to make a query to bring these results. I know that sum would bring me the total amount for each customer but I don't know how to specify which fields are supposed to be negative so that it subtracts them instead of adding them.

By the way I'm not allowed to change the structure of the table.

Could you help me try to create this query?you need to use a CASE stmt. check out BOL for some samples.

hth|||Could you give me an example through this thread please. I have no access to other internet pages. I can only access this one.

I tried something like this but it didn't work out

select r.numpolso, r.numofic,
case r.tdocumen
when 'N' then sum (r.prmtotrb*'-1')
when 'R' then sum (r.prmtotrb)
end as r.prmtotrb
from trecibos r
where r.numpolso = '1010' and r.ramo = '90' and r.subramo = '14' and r.numofic = '001'
group by r.numofic, r.numpolso, r.prmtotrb

I got a syntax error msg

could you give me an example or another way of doing this?

thanks in advance