You could put
=IF(MOD(ROW(),2),(ROW()+1)/2,"")
in row 1 and copy it down
(or the opposite
=IF(MOD(ROW(),2)=0,ROW()/2,"")
Mod simply returns the remainder. For example in row 3, it would return a 1 since 2 goes into 3 once with a remainder of 1. This is just used to check for even or odd rows. Excel interprets 0 as false, and a positve number as true.