Adding additional column to get headcount for each month in OTBI

Summary:

How to add additional column or calculated item to get the head count for every month?

Content (required):

I am using 'Workforce Trend Real time Subject Area' and used 'Period End Headcount' to get the headcounts. I have used Pivot table view to get desired output. The challenge I am facing is I need to add a total count of employee in every month. How can it be achieved? below is expected result and my current OTBI output.

Expected result: Below is the result needed

OTBI I have prepared: Below is what i fetched, I need to add 4th column under every month which should be the sum of first 3 columns of month for each row. And also need the SUM of this 4th column in the bottom.