Formula for only adding cells when both cells contain data

Hi,

I am struggling to find the right solution to my problem.

I am pulling data from another sheet, to get prices for lots.
Lot 1, lot 2 and a total (lot 1 + Lot 2) for Lot 3.

However, if they only complete Lot 1 Costs-Lot 3 needs to remain Blank, the same with Lot 2 (Lot 3 remains Blank). However if they complete both Lot 1 & 2, then lot 3 needs to show the total cost for both (Lot 1+Lot2)

Any wise help would be really appreciated.

MMB0342