Upgrade 2011 Freelance Plus -> 2014 -> 2015 in one or two steps?

I'm currently using Studio 2011 Freelance Plus, bought the respective upgrade to Studio 2014 a year ago, then the upgrade from 2014 to 2015  but ... have been to busy to get any of them installed until now.... Do I have to upgrade in two steps, first to 2014, then to 2015? Or is it possible to proceed in one single step right from 2011 to 2015?