import java.util.Scanner

object Main extends App {
  val sc = new Scanner(System.in)

  val s = sc.next

  val notFound =
    Set('a','b','c','d','e',
        'f','g','h','i','j',
        'k','l','m','n','o',
        'p','q','r','s','t',
        'u','v','w','x','y','z') &~ s.distinct.toSet

  val res = if(notFound.isEmpty) "None" else notFound.toList.sorted.head

  println(res)




}