ADhoc Difference Calculation based on Column Group

[itgroup]

Hi, We have a dataset like this with the first 3 columns.  We want to add the 4rth calculated field "Difference".  Seems simple, but can't make it work.  The years (2015, 2016) are part of column measures ... so potentially there would be more than 2 ... which I assume is what makes this hard.  Any ideas?

Thanks!

Customer	2015	2016	Difference
Customer A	500	600	100
Customer B	1000	800	-200