Excel, when pasting formula down the page last number duplicates itself?

What I am trying to do is a formula for the whole page (not sheet, just 1 printable page). I need to deduct a2 from b1, a3 from b2 etc. But when I do the formula for the whole page the last sum duplicates all down the page. But I want the formula to be there, but the cells to be empty unless I have input a figure to be deducted. Otherwise blank, not duplicating the last sum all down page. Does that make sense?