import java.util.*

fun main(args: Array<String>) {
  val sc = Scanner(System.`in`)
  val s = sc.next()

  var has = true
  var ans = ""
  for (i in 'a'..'z') {
    val c =  i.toChar()
    if (s.contains(c)) {
      continue
    } else {
      has = false
      ans = c.toString()
      break
    }
  }
  if (has) ans = "None"

  println(ans)
}
