let (|One|_|) x = if x = 1 then Some x else None

let (|Even|_|) x = if x % 2 = 0 then Some x else None

​

let lenth x = 

   let rec calc num pos =

      match num with

      | One i -> 1, pos

      | Even i -> calc (i / 2) (pos + 1)

      | _ -> calc (num * 3 - 1) ( pos + 1)

   let i pos = calc x 0

   i

printfn "%i" (lenth 10)