Scanf.scanf "%d %d %s" (fun n k s ->
    print_endline @@
        String.init n (fun i ->
            if i <= n - k then s.[i + k - 1] else (
                if (n - k) mod 2 = 1 then s.[i - n + k - 1]
                                     else s.[n - i - 1]
            )
        )
)