Formula field returns only 0

Question

Hi,
I have a Crystal Report which uses data from a Data Table.
I have  grouping and calculating sum within the groups. The grouping is by the field  vid.

I want to perform calculation where if vid="Pri" the field izn of  that row should be summarized, and if vid="Isp" then the field izn should be  summarized and then subtracted from the first sum (when vid=Pri).

For the  first sum (if vid="Pri") the calculation is ok (for the group with vid="Pri"),  but if vid="Isp", (for the group with vid="Isp"), the result of the sum function  is always 0, i.e. it doesn't calculate correctly. So I can not perform the last  calculation (summarize when vid is Pri and subtract the sum, when  vid=Isp).

The first formula which calculates correctly  is:

if({DataTable3.vid}="Pri") then Sum ({DataTable3.Iznos},  {DataTable3.vid})

and the formula for the other sum, when vid=Isp  is

if({DataTable3.vid}="Isp") then Sum ({DataTable3.Iznos},  {DataTable3.vid})

Can anybody help me please what is the mistake I make  in the bold line for having the result 0, instead of the correct  result?

Below is the image of the report

Thank you in advance