How to get the difference between two columns in a column group

Hi All,

My first time here and really new to programming. I would like to get the difference between 2 columns that are inside 

a column group.

Here is my sample table below: The Column Group is PeriodNumber and can only choose 2. like 1 and 2.. I would like to have a third row which will simply calculate the difference between the amounts in PeriodNumber 1 and 2.

                            PeriodNumber          

Account                    1                            2     

1) Cash                10,000                15,000

2) Receivables      12,000                11,500

3) Equipment          5,000                  5,500

Total Assets          27,000                32,000