fun readInt () =
    valOf (TextIO.scanStream (Int.scan StringCvt.DEC) TextIO.stdIn)

val () =
    let
        val b_s = List.tabulate (9, fn _ => readInt ())
        fun findAns (h::tl) i =
            if h = i then findAns tl (i + 1)
            else i

        val ans = findAns b_s 1
    in
        print ((Int.toString ans) ^ "\n")
    end