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


exception BadInput
fun findAns [] _ = raise BadInput
  | findAns _ [] = raise BadInput 
  | findAns (h1 :: tl1) (h2 :: tl2) =
    if h1 <> h2 then (Char.toString h2 ^ "to" ^ Char.toString h1)
    else findAns tl1 tl2
                
                    
val () =
    let
        val s = readStr ()
        
        val ans = findAns (String.explode s) (String.explode "abcdefghijklmnopqrstuvwxyz")
    in
        print (ans ^ "\n")
    end