Need a function that made me the sum if isna (is not N/A)

[Cata_bRc]

Hy I have a ldata base with the next fields: tv, fridges and computers with their pices. All that information is bringed in the front page by data validation. I make the sum of the prices. Until this all its ok. But if I want to buy only a fridge and a computer and not a tv the sum will look #N/A
How can I resolve this problem: to have the sum even I didn't selected all the parameters?
Thank you very much.