Report through Sharepoint list in SSRS while using group function

Hello,

I have a list in sharepoint with the following detail.

1. Accountant
2. Customer_Id
3. points1
4. point2

All of the above fields are more then one record. I want to display record like that

1. Accountant (Distinct / Group by)
2. Customer_Id (Count(Customer_id) according to accountant
3. Points count(point1 + point2) according to accountant

But the problem is when I do sorting according to that 

count(point1 + point2) in tablix properties like that:

=reportitems!Textbox3 (That expression is in Text3)

SSRS gives me error and that is "Report Item cannot be used in sort expression"

How can I sort data according to expression?