Expander View in Windows Phone

Question

I have used an expander view in my app.It  contains sublistItems.The sublistitems could be one or many.

For example if the expander list headers  are 1,2,3.The sublistitems for 1 is "a","b","c".

I have to get the index of the subitem(ex: "a"or"b"or "c")when clicked.