Problem with sumarizing only certain rows in the footer of the crystal report

Hi,
 
Can anybody help me please how to summarize with formula field only those rows in the report footer of the crystal report which have certain value?
 
For example I want to summarize by the field "Iznos" only those rows with value "Isp" for the field "vid". Something like:
 
for vid="Isp" sum(Iznos)
 
I have this table
 
brdok Iznos     vid
 
29     453        Isp
 30     2152      Pri
 37    215,6      Pri
 38     548        Pri
 
In the first group

Isp
 
            sum=453
 
for the second group again the sum is correct
 
Pri  

           sum=2915,6
 
So, in the report footer I want to use the sums of the both groups and to have as results for the formulas
 
{@Pri}= 2915,6  and
 
{@Isp}= 453
 

Thank you very much in advance