fun readLargeInt () =
    valOf (TextIO.scanStream (LargeInt.scan StringCvt.DEC) TextIO.stdIn)


fun int_to_digits n =
    let
        fun int_to_digits_aux nn l =
            if nn > 0 then int_to_digits_aux (nn div 10) ((nn mod 10) :: l)
            else l
    in
        int_to_digits_aux n []
    end


fun findAns n =
    let
        val digits = int_to_digits n
    in
        if List.length digits = 1 then List.hd digits
        else findAns (List.foldl (fn (d, acc) => d + acc) 0 digits)
    end


val () =
    let
        val n = readLargeInt ()

        val ans = findAns n
    in
        print (LargeInt.toString ans ^ "\n")
    end