VS 2010 Help finding biggest number from 2 list boxes and displaying it in 3rd list box?

Hi guys, im really stuck and i dont know how to do this, so a code will be highly appreciated.

I have 2 list boxes each filled with 10 numbers.

1. I want when i click a button to display the biggest number from the 2 boxes in 3rd one.
Example:
1 - 2 -> 2
3 - 4 -> 4

2. And if in the some of the list boxes contain less numbers than the other one, in 3rd to be displayed the number from the other.
Example:
3 - x -> 3

This is it..
Attachment 95451

Sorry for my bad english, and sorry the buttons are in Bulgarian!
Anyone can help?

Attached Images