Formula conditional on contents of cell

[data_pig]

I have a simple calculation that I want completed BUT only when there is a number entered in to a certain cell.

Simply I want =A2-A1 and the answer to show in cell A27, BUT only when cell A2 has something in it. If cell A2 is blank I don't want to have the calculation happen (or to produce a non numerical answer like n/a or null or something). I'm sure I have done this, or something similar, before but I've just hit a brick wall trying to get it to work! If anyone has any suggestions please help.

[NeedForExcel]

Maybe -

=IF(A2<>"",A2-A1,"")

But just a note, even if A2 is blank, and you are trying =A2-A1, it should not return an error.. It will calculate the blank as if it were 0

[samba_ravi]

=IF(A2<>"",A2-A1,"")
or
=IF(ISNUMBER(A2),A2-A1,"")

[data_pig]

=IF(ISNUMBER(A2),A2-A1,"")

Worked perfectly!!! Thanks so much.