import java.util.*

fun main(args: Array<String>) {
    val sc = Scanner(System.`in`)
    val s: String = sc.next()
    println(answer071bAlt(s))
}

fun answer071bAlt(s: String): String {
    val alphabets: Array<String> = arrayOf(
        "a",
        "b",
        "c",
        "d",
        "e",
        "f",
        "g",
        "h",
        "i",
        "j",
        "k",
        "l",
        "m",
        "n",
        "o",
        "p",
        "q",
        "r",
        "s",
        "t",
        "u",
        "v",
        "w",
        "x",
        "y",
        "z"
    )
    val set: MutableSet<String> = mutableSetOf<String>()
    for (c: Char in s) {
        set.add(c.toString())
    }
    for (i: Int in alphabets.indices) {
        if (!set.contains(alphabets[i])) {
            return alphabets[i]
        }
    }
    return "None"
}