import java.io.InputStreamReader
import java.io.BufferedReader

fun main(args: Array<String>) {
  val br = BufferedReader(InputStreamReader(System.`in`))

  val ref = "abcdefghijklmnopqrstuvwxyz".toCharArray()
  val s = br.readLine().toList()

  val res = ref.subtract(s).firstOrNull()
  val ans = if(res != null) res.toString() else "None"

  println(ans)
}