Minecraft Junkie

I'm a little stuck here guys. I bought a steam card from gamestop recently because my parents won't pay online so I thought to take matters into my own hands. Now, I have already purchased ARMA 2, but DayZ requires ARMA 2: Combined Operations, which is a combination of ARMA 2 and ARMA 2: Operation Arrowhead.

So basically, can I just buy Operation Arrowhead on its own, to save money?
Or
Would I have to purchase Combined Operations?

I just don't want to make any silly mistake because I really want to play DayZ

The combined ops are Arma2 and op arrowhead. Which is cheaper of u buy them together but u only need operation arrowhead now so just buy arrowhead.