[EpiData-list] sum of two variables automatically

This may be a stupid question, I used the new version of Epidata to create a database and I want to create an entry (V3)  the sum of the previous two entries (V1 + V2): V1 = x V2 = y V3 = sum (V1 + V2) Can someone give me the procedure or point me to a good tutorial? thank you very much = ps files are in format (. epx)