Greetings all

I'm trying to get excel to calculate a formula for every other row is this
possible.

Example:
Column A1-A9 has 1 thru 9 in it.
Column C1 has the formula A1+3 and I would like that formula to be in every
other row of C. So C1 would be A1+3 ,C2 would be blank, C3 would be A2+3,
C4 would be blank, C5 would be A3+3 and so on.

Is this possible?
tia sal2