Permutations are elements of symmetric groups. Since in Project P, we always adopt one-line notation unless otherwise stated, a permutation of length `n` can be treated as a sequence of `{1,2,\cdots,n}`.
For example, `123`,`132`,`231`,`213`,`312` and `321` are the `6` permutations of length `3`.
`a(n)` is the number of permutations of length `n` such that numbers at odd positions are monotone and numbers at even positions are also monotone. For example, `a(3)=6` and `a(4)=24`.
Find `a(n)`.