formula - Conditional group SUM in Crystal Reports

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C#3.5

Report Display like this
1/5/2015 Opening Balance 184.00
1/5/2015 Receipt 500.00
1/5/2015 Issue 184.00

Total 500.00

2/5/2015 Opening Balance 184.00
2/5/2015 Receipt 500.00
2/5/2015 Issue 584.00

Total 100

How to get the result. i have write the formula

Global NumberVar MontQty;

if {Ledger_item.type} = "Receipt" then
    MontQty := MontQty + ToNumber({Ledger_item.quantity})
else if {Ledger_item.type} = "Opening Balance" then
    MontQty := MontQty + ToNumber({Ledger_item.quantity})
else
    MontQty := MontQty  - ToNumber({Ledger_item.quantity})

it is not working . first group is ok. second group total value get 600(500+100). i want only 100