Difference between 2 values in a Matrix Table

Hi, 

I am trying to create a matrix column (diff) that holds the values of what the difference is between two year columns. Example:

2015        2016            *Diff

________    _______        ______

10              6                -4

23             15               -8

187           195            +8

25             30              +5

What measures would I be using? I created these:

2015 = CALCULATE(COUNT('Phone Leads'[Calls]), Dates[year] = "2015")

2016 = CALCULATE(COUNT('Phone Leads'[Calls]), Dates[year] = "2016")

Diff in Years = [2016]-[2015]

But I'm getting this error message:

Any advice?