object Main extends App {
	import scala.io.StdIn._
	
	val S = readLine
	val A = "abcdefghijklmnopqrstuvwxyz".toList.reverse
	var r: String = ""

	for(i<-A){
		if(! S.contains(i.toString)) r = i.toString		
	}

	if(r == "") println("None")
	else println(r)
}