fun main(args: Array<String>) {
    val s = readInputLine().toCharArray()
    s.sort()
    val s2 = s.distinct()

    for ((i, c) in ('a'..'z').withIndex()) {
        if (i >= s2.size || s2[i] != c) {
            println(c.toString())
            return
        }
    }

    println("None")
}
 
fun readInputLine(): String {
    return readLine()!!
}
