Calculating sum on previous selection year month

[1karan]

I have a Table and it has selection year month field (which is the filter year month field). I want to calculate the Current amount based on the previous Year month figure in the current column. i.e. for Selection year month 202101 it should show me that period SUM(current figures)

I think we can use before function based on rowno but it is not giving any result.  What should be the correct expression which we can use