Scanf.scanf "%s" (fun s ->
    let n = String.length s in
    let rec loop i prev =
        if i = n then "NA" else
            let prev = if s.[i] = 'O' then (
                if prev >= 0 then prev + 1 else 1
            ) else (
                if prev <= 0 then prev - 1 else -1
            )
            in
            if prev = 3 then "East" else
            if prev = -3 then "West" else
                loop (i + 1) prev
    in
    loop 0 0 |> print_endline
)