Addition of Multiple selected index in listbox

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C#

C#4.0

Hi,I have listbox(selection mode) that contains n number of data.I`m converting selected index (0,1,2,3...) into (1,2,4,8...) respectively using pow()method and again decoding using log2()method..

Now if i select 2 data in listbox say(1,2) then i should able convert into(2+4=6).Please help me how to convert when multiple data selected??