Storing Part Of Listbox Items Into A String?

I have a listbox where the user can add items to it with the following schema "count, id".I want to take each item from the listbox,use the split function so I can "take" only the "count," part and then add the "count," part of each item into a string.I mean I want something like this:

user adds 3 items in the listbox:

1, 100
2, 200 and
3, 300.

I want to make it,so a string can be formed, and contain the "count," part of each item which in this case is;

in the first item the "count," part is "1,",in the second item the "count," part is "2," and in the third it's "3,".I want those "count," parts to be stored into a string (which will result into having this string "1,2,3,").