`a(n)` is the number of lattice paths from `(0,0)` to `(n,2n)` that is not above the line `y=x`.
For example, a path from `(0,0)` to `(5,10)` not above `y=x` is illustrated in the following picture.
Find `a(n)`.
Updated 03/01/2015
Typos are fixed.