Getting the index of the selected option in Listbox

[revosw]

I supply items in the Listbox that is circular, meaning option 1, 2, 3, 4, ... appears again later at 11, 12, 13, 14, ... for example. This means I don't know whether I have selected option 1 or 11, but it is significant whether I selected option 1 or 11. I tried looking for a way to get which index is selected in Listbox. How have you all handled this case?