how to put condition in rdlc column addition ?

Question

i have 2 coumn in my rdlc  

naration and amount as shown below

 

naration	amount
opening balance	100
closing balance	100
opening balance	100
closing balance	100
total	400

 

in this the amount column is calculating all the record as total, which i dont want 

 

i want to add the amount without taking naration as  opening balance amount, rest all should be added as shown below

 

naration	amount
opening balance	100
closing balance abc	20
opening balance	100
closing balance xyz	30
total	50

 in above eg it added the cloumn without taking opening balance 

can we write a expression in rdlc which will do the same.