import java.util.Scanner

object Main {
  def main(args: Array[String]): Unit = {
    //nextLineとnextを同時に使ったら死ぬ
    val sc = new Scanner(System.in)
    val s = sc.next()
    val ans =
      ("abcdefghijklmnopqrstuvwxyz".toCharArray.filter(m => !s.contains(m)))
    if (ans.length == 0) {
      println("None")
    } else {
      println(ans.head)
    }
  }
}
