Routine in Bex - urgent pls.

I have 2 columns A and B in the report:

A B C

-

1234 1234 1234

5903 0 5903

0 3287 3287

I want to get the column C.

So, the condition for the column C would be :

if A<>0 and B <>0 then A

else if A==0 then B

else if B==0 then A

I heared that this can be done by routines . I need the code to do this . Also let me know where to put the code.

This is urgent pls.

Vaishali