import java.util.*

val sc = Scanner(System.`in`)
val S: String = sc.next()

fun main(args: Array<String>) {
    println(judge(S))
}

fun judge(s: String): String {
    for (c in 'a'..'z') {
        if (!s.contains(c)) {
            return c.toString()
        }
    }
    return "None"
}
