Calculation for a parameter in crystal reports

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CrystalReports

hi..

I cam across this problem and need a solution.

I have a crystal report which generates a bill..
In the bill it includes the amount for the item and the charges that applies to the amount.

If the charges have description "ADD" , it must be added to the item total and if it has description "LESS" it should be deducted from item total and final amount should be displayed as total.

The report displays all the data, I tried to take a variable named total and on its field explorer i wrote this..

int _total;
if({Expensedetail.add_less}='add')
{
_total=_total+{Expensedetail.Amount};
}
else
{
_total=_total-{Expensedetail.Amount};
}

this is not working...can you guys please suggest me what to do??