[C#] How to plus two columns as the third column in CRV ?

I have two columns INTEREST and  CAPITAL in Crystal report which each column has multiple rows. I need INTEREST+CAPITAL=third column as named TOTAL row dend on INTEREST and  CAPITAL; 

 

e.g:

 

INTEREST   +    CAPITAL      =    TOTAL 

200                     800                   1,000     

100                     600                    700       

 

The total column is the Crystal Report formula Field and two other is from database or data set.

 

How can I do this ? Any solution please share to me.

 

Advance thanks.