43132
Ok, but how do we get to this number? Now it's your turn, because your solution is probably better than mine!
I haven't been able to find a simple formula for this one, not even with an ellipsis (dot-dot-dot).
If you are able to find a closed-form solution, please e-mail it to me (waltertross ∧ gmail · com),
and I will publish it here together with your name or pseudonym.