Need someone to help me with this sum

Question

Hi every one i need help really quickly. I have a deadline for tonight and need some to help me do a sum in c#.

In the textbox i have to enter values for example "5,5,10,10,7,7,9,9,20,20,3,3,4,4" and it has to take the first entered and the 3rd last and last entered and plus them together