object Main {
  def main(args: Array[String]): Unit = {
    import scala.io.StdIn.readLine
    val instr = readLine.split(" ").head
    val alphabet = "abcdefghijklmnopqrstuvwxyz"
    val ans = alphabet.filterNot(e => instr.contains(e))

    println( if(ans != "") ans.head else "None" )
  }
}
