How do I get a blank cell when copying formula's down the page

[BBTrekkie]

I have the following formula I use for doing inventory.

Column A is for incoming so it gets added from the answer in the line above
COlumn B is for outgoing so it gets subtracted from the answer in the line above

Now I want to copy the function down the page, but when I do I get the last answer copied all the way down. I just want it to appear blank when there is nothing in column A or B until I add figures.

Incoming Outgoing Inventory
500 500
100 400

as a very simple visual example

=IF(A3>0,C2+A3,C2-B3)
Now when I copy this down the colum it get the 400 going all the way down. I just want it blank if there are no numbers in a or b. I played with this for over an hour using help and got nowhere.

Thanks so much.

[kraljb]

=if(and(a3="",b3="'),"",if(a3>0,c2+a3,c2-b3))