fun readStr () =
    let
        fun scan reader stream = SOME (StringCvt.splitl (not o Char.isSpace) reader (StringCvt.skipWS reader stream))
    in
        valOf (TextIO.scanStream scan TextIO.stdIn)
    end


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


val () =
    let
        val s = readStr ()
        val e = readStr ()
        val n = readInt ()
       
        val ans = if n = 1
                  then
                      (
                        if s = e then s
                        else "-1"
                      )
                  else
                      (
                        s ^ String.substring ("aaaaaaaaaa", 0, n - 2) ^ e
                      )
    in
        print (ans ^ "\n")
    end