Formula Field Calculation Of Crystal Reports

Question

I have a crystal reports which containing information of Items calculation as below
date    open bal  inward  outward   balance
01/04/2012  10     NIL       NIL
02/04/2012  NIL     2         NIL 
03/04/2012  NIL    NIL        5

The column balance is inform of formula field and I wants to calculate balance formula field as below.
date    open bal  inward  outward   balance
01/04/2012  10     NIL     NIL           10
02/04/2012  NIL     2       NIL           12 
03/04/2012  NIL    NIL      5             7

How to calculate formula field balance?.