Problem with the formula field in the Crystal Report

Question

Hello,

I have two formula fields in the report footer in my Crystal Report.
The both values should depend on the value of another field. Fo r example if {DataTable3.vid}="Pri" it should take the value of Iznos from the table for the rows where vid is "Pri" and if vid ="Isp" it should tkae the value of Iznos for that rows of the datatable where vid = "Isp"

Here is my formula for the first formula field, which works fine:
if({DataTable3.vid}="Pri") then Sum ({DataTable3.Iznos}, {DataTable3.vid})

and the value for the second formula field, which doesn't function ok:
if({DataTable3.vid}="Isp") then Sum ({DataTable3.Iznos}, {DataTable3.vid})

The second returns 0.00

Can anybody help me please?
Thanks in advance