yes, it should be possible.
What Sunny did, was to change your formula using what is called "Set Analysis".
I too think, that this should do the trick.
However I still have a question: Do you have full date values (including day) in the field called [Month]?
I ask, because if this would be the case, I find it most likely, that you propably might not have a [Date] field, as suggested for use as in Sunny's example and might have to adapt it a little bit by changing "Date" to "Month". But if you have a [Date] field, you will do just fine.