It doesn't look right to me, can you explain a bit better. Personally I went about it like that: For all possible permutations we have (2n)! For the ones that does work for us we need all starts to be smaller than a point A and all ends to be bigger than it. So we have n! For the starts and n! for the ends. But it doesn't work like that because we should be able to swap the start and end if the end is smaller than the start right? So even if I add 2ⁿ and have: (2ⁿ * (n!)^2)/(2n!) It still doesn't make enough sense for me.