import scala.io.StdIn

object Main extends App {

  val s = StdIn.readLine()
  val a = Array.ofDim[Int](26) // a to z
  
  s.foreach(c => a(c - 'a') += 1)
  val r = a.indexWhere(p => p == 0)

  //println(a.mkString(" "))
  
  println(if(r < 0) "None" else (r+'a').toChar)

}
