object Main extends App {
  def check(s:Set[Char]) = {
    val s_ = (for(c <- 'a' to 'z' if !s.contains(c)) yield c).toArray
    if (s_.length == 0) None
    else Some(s_(0))
  }
  val s = readLine.toSet
  println(check(s) match {
    case Some(c) => c
    case None => "None"
  })
}