How to use sum() then find difference of the sum()'s result in separated two columns in Crystal Report?

 Have a stored proc which populated 8 columns. so I need to display this output in a crystal report. I am able to populate all 8 columns in report perfectly. but my next task is to add a column which has sum of column 8th on the condition plus grouping and then find the diff of the sum's result like:

column1    column2   column8   Sum      diff

a                dd            12

a                dd            11             23

a                ee            33            33       -10

b                dd            25            25    

b                cc             10           

b                ss            22            32         -7

c                aa            30            30         30

As my understanding here i need to use grouping in coulumn1 and then use where condition on columns2.

Anyone has any idea how to print report like this.

Thanks in advance.