display addition of credit and debit amount at end of column

hi experts

i am developing a report where i want addition of credit amount and debit.

i am trying but the addition is not correct.

op is

gl no ,disc credit ,debit.

eg.

gl_no description credit debit

1232 khgkhdflhg 345 456

1234 ghgkgkdfh 788 555

1237 jmnkbnbbl 873 498

- -

? ?

i want to calculate the total credit and debit amount.

thanks

ajay