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()
    println(
      "abcdefghijklmnopqrstuvwxyz".toCharArray.filter(m => !s.contains(m)).head
    )
  }
}
  