
EXAMPLE

For n = 5, the possible matrices are [3,5,1,2], [3,1,5,2],
[3,1,5,2], [3,5,1,2], [2,4,1,2], [2,1,4,2], [2,1,4,2], [2,4,1,2], [1,1,3,2], [1,3,1,2], [2,5,1,3], [2,4,1,2],
[2,1,5,3], [2,1,4,2], [2,1,3,1], [2,1,4,2], [2,1,5,3],
[2,3,1,1], [2,4,1,2] and [2,5,1,3].
Here each of the matrices is defined as M = [a,b,c,d] where a = M[1][1], b = M[1][2], c = M[2][1], d = M[2][2]. There are 20 possibilities. So, for n = 5, a(n) = 20.
