object Main {
    def main(args: Array[String]) = {
        val sc = new java.util.Scanner(System.in)
        val s = sc.next
        println(calc(s))
    }
    def calc(s: String) : String = {
        val alphabet = "abcdefghijklmnopqrstuvwxyz"
        val ret = alphabet.filter(x => !s.contains(x))
        if (ret.length > 0) ret(0).toString else "None"
    }
}