Hi I'm having some problems with the following question!!

Show S2n,2 = 2n + (2n 2) + (2n 3) + ..... + (2n n-1) + 1/2(2n n)

[Note that (2n 2), (2n 3) etc are the notation for the binomial coefficients]

I'm having trouble understanding how to expand a stirling number. I know the general forumla but if anyone knows how to make it look like the one above that would be great!

Thanks