module NthPrime

let isPrime n = 
    let stop = int(sqrt(float(n)))
    seq { 2..stop }
    |> Seq.exists (fun x -> n % x = 0)
    |> not

let oddNumbers = Seq.initInfinite (fun i -> i + 2)

let oddPrimes = Seq.filter isPrime oddNumbers

let safeNthPrime n = if n = 1 then 2 else Seq.last (Seq.take n oddPrimes)

let prime nth : int option = if nth = 0 then None else Some(safeNthPrime nth)