how to find checkboxlist first selected item

Question

i wanted to know how do u get checkboxlist selected item ie, if checkboxlist contains 6 items say c1,c2,c3,c4,c5,c6. if i select c4,and c1. i wanted selected index as 4,1 i,e i wanted which is first selcted index,second selected index and so on. if run loop i can get items selected in checkbox list as 1,4. but iwanted which is selected first,second and so on

Help is aprreciated