Compare 2 months using Slicer

Hi,

i have a table where i need to compare values from 2 months using slicer, at this moment im using this measure:

Total LM = CALCULATE([nStudents];PREVIOUSMONTH('Month'[Data]) ) when i try to use this measure i dont get any result.

Sample Data

Data                        ClassName     nStudents       Total LM

01/01/2018               ClassA                30                  

01/02/2018               ClassA                25                  -5

My Month[Data] its already set as data table if it helps.

At my relationship i have

My class table as * to 1 for Month table.

My class table has IdMonth .

Rui Silva