
object Main extends App {
  val sc = new java.util.Scanner(System.in)
  val s = sc.next.distinct.sorted
  val as = "abcdefghijklmnopqrstuvwxyz"

  val ss = s + "*" * (26 - s.size)
  val q = (ss zip as).find(c => c._1 != c._2)
  println(if (q.isEmpty) "None" else q.get._2)
}