object Main extends App {
  
  val sc = new java.util.Scanner(System.in) 
  val s = sc.next().toList.distinct.sorted
  val a = ("abcdefghijklmnopqrstuvwxyz").toList
  val ans = a.zip(s).find{case (x, y) => x != y}.map{case (x, y) => x}.getOrElse("None")
  println(ans)
}