How to summrize columns in crosstab report

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CrystalReports

friendz am stuck at a point that in crystal Crosstab report, i need to have sum of only two column's value and subtract the total by the value what should i do?

as in below,

        Product 1    Product2    Product 3    Total
        ----------------------------------
Cust 1      4           5           2          11
Cust 2      5           9           7          19

I want to sum values of "Product 1","Product 3". then subtract from row total=> Total LIKE [4+2]-11=>TOtal. and want to repeate this for each customer..

the way which I using to solve is.....[this is basicly C.R formating Display String Formula with VB Syntax]

Global totprodsum as int

Global Ccust As String 
Dim Ldept As String 
Whileprintingrecords
if (Ccust="") then
    totprodsum=0
    totprodsum=totprodsum+currentfieldvalue
    Ccust=gridrowcolumnvalue("Table_Customer.Name")
    formula=currentfieldvalue
else
    if Ccust<>gridrowcolumnvalue("Table_Customer.Name") then
        totprodsum=0
        totprodsum=totprodsum+currentfieldvalue
    else
        totprodsum=totprodsum+currentfieldvalue
        formula="Newag"
    end if
formula=currentfieldvalue

end if

Ccust=gridrowcolumnvalue("Table_Customer.Name")

but its not working bacause Crystal report generates report in this sequence

Cust 1 <1st col value>

then.....

Cust 2 <1st Col Value>

then..........

Cust 1 <1st col value> <2nd Col Value>
Cust 2 <1st Col Value>

then ....

Cust 1 <1st col value> <2nd Col Value>
Cust 2 <1st Col Value> <2nd Col Value>

then .....

Cust 1 <1st col value> <2nd Col Value> <3rd Col Value>
Cust 2 <1st Col Value> <2nd Col Value>

then..

Cust 1 <1st col value> <2nd Col Value> <3rd Col Value>
Cust 2 <1st Col Value> <2nd Col Value> <3rd Col Value>

while I want crystal report to Execute report like this

Cust 1 <1.1> <1.2> <1.3>

then......

Cust 2 <2.1> <2.2> <2.3>


anyone please help me out.