Rolling Sum

Hi Everyone,

I am fairly new to Power BI and struglling a lot to understand how to add a calculated column that does rolling sum based on Month (String), Cost Center and Cost Element. I have a table like below:

So based on Cost Center (CC), Cost Element (CE) and Month Name I want to do rolling sum of Actuals. E.g. YTD Sum of CC 101 and CE 201 on Feb-15 was 22 (10+12). Since it is year to date rolling sum Jan 15 number doesn't depend on Dec-14 but also need to consider this as I have data from 2013 to 2016.

How do I make this happen. Can anyone please suggest a formula?

Really appreciate your time.

Best,

Keyur Parekh