Formula only active if cell blank

I want to show variance (i.e. =A2-A3), but I only want it to calculate if the cell is showing a number (i.e. A3)

so-
A2= 1 and A3= 1, then A4=2
but if-
A2=1 and A3= blank, then A4=blank (rather than -1)

Thanks for any help on this presumably basic matter...

=IF(A3="","",A2-A3)

HTH

Jason

You also might want to validate before making the calculation (in case there
are letters rather than numbers in some of the cells):

=if(and(isnumber(A2);isnumber(A3));A2-A3;"")