How to Get Difference Between two Field in Select Mode

Dear Experts,
Please help me in getting the difference Quantity Between Two Quantity Fields in Select Mode.
Thank You.

Code:

[#Line  : DB Title]
Add:Right Fields:Before:DSP MainDateTitle:BillSelectedQuantity, BillsSelectedActQty, Difference
[Field : BillSelectedQuantity]
  Use : Name Field
  Set as  :if @@InselectMode Then $$String:@SelectedTotal + $$String:@SelectedTotalQty Else ""
  Width  : 20
  Align  : Centre
  SelectedTotal    : $$LocaleString:"Bills Selected for : "
  SelectedTotalQty  : $$String:##SelectedLineVariable2
  Invisible  :  ##IsVoucherregister
  [Field : BillsSelectedActQty]
  Use : Name Field
  Set as  :if @@InselectMode1 Then $$String:@SelectedTotal + $$String:@SelectedTotalQty  Else ""
  Width  : 20
  Align  : Centre
  SelectedTotal    : $$LocaleString:"Actual Qty :"
  SelectedTotalQty  : $$String:##SelectedLineVariable3
  Invisible  :  ##IsVoucherregister
  [Field : Difference]
  Use : Name Field
  Set as  :if @@InselectMode1 Then $$String:@SelectedTotal + $$String:@SelectedTotalDiff else ""
  Width  : 20
  Align  : Centre
  SelectedTotal    : $$LocaleString:"Difference :"
  SelectedTotalDiff  : $$String:@@Difference
  Invisible  :  ##IsVoucherregister
  [System:Formula]
  Difference: #BillsSelectedActQty-#BillSelectedQuantity

 

Convert the qty in number and Subtract it

Example
Set as $$Number:$ActualQty) - ($$Number:$BilledQty)

 

Thank You. I solved it by using this Set as$$Number:##SelectedLineVariable3) - ($$Number:##SelectedLineVariable2).

 

How Do I make Field: Difference Invisible if the There is no Value in Actual Qty Field.
I have tried.
Invisible: $$IsSales:##VoucherTypeName Or $$IsEmpty:##SelectedLineVariable3
Invisible: $$IsSales:##VoucherTypeName Or $$IsEmpty:$ActQty


[#Line : DB Title]
Add:Right Fields:BeforeSP MainDateTitle:BillSelectedQuantity, BillsSelectedActQty, Difference

[Field : BillSelectedQuantity]
Use : Name Field
Set as :if @@InselectMode Then $$StringSelectedTotal + $$StringSelectedTotalQty Else ""
Width : 20
Align : Centre
SelectedTotal : $$LocaleString:"Bills Selected for : "
SelectedTotalQty : $$String:##SelectedLineVariable2
Invisible : ##IsVoucherregister
[Field : BillsSelectedActQty]
Use : Name Field
Set as :if @@InselectMode1 Then $$StringSelectedTotal + $$StringSelectedTotalQty Else ""
Width : 20
Align : Centre
SelectedTotal : $$LocaleString:"Actual Qty :"
SelectedTotalQty : $$String:##SelectedLineVariable3
Invisible : ##IsVoucherregister
[Field : Difference]
Use : Name Field
Set as: if @@InselectMode Then @SelectedTotal + @SelectedTotalDiff else ""
Width : 20
Align : Centre
SelectedTotal : $$LocaleString:"Difference :"
SelectedTotalDiff : $$String($$Number:##SelectedLineVariable3) - ($$Number:##SelectedLineVariable2))
Invisible : $$IsSales:##VoucherTypeName Or $$IsEmpty:$$Value:##SelectedLineVariable3

 

ok Solved by Using this. Thank You.
[Field : Difference]
Use : Name Field
Set as: if @@InselectMode Then @SelectedTotal + @SelectedTotalDiff else ""
Width : 20
Align : Centre
SelectedTotal : IF $$IsEmpty:##SelectedLineVariable3 Then "" Else $$LocaleString:"Difference :"
SelectedTotalDiff : IF $$IsEmpty:##SelectedLineVariable3 Then "" Else $$String($$Number:##SelectedLineVariable3) - ($$Number:##SelectedLineVariable2))
Invisible : $$IsSales:##VoucherTypeName