Calculate sales difference from previous years in Analytics

Hi all,

Here's an over simplified example of what I'm trying to do in Analytics:

I've got total ticket sales broken out by season (that part is easy and I have been able to get that far in Analytics). What I can't seem to figure out is column C: how to get a calculation that compares a season's sales with the previous season. Is this something that is even possible?

Thanks,

Sara