fun main(args: Array<String>) {
    val iv = readLine()!!.toCharArray().distinct().sorted().map { it - 'a' }
            .withIndex().reduce { acc, iv -> if (acc.index == acc.value) iv else acc }

    val ch = 'a' + iv.index + if (iv.index == iv.value) 1 else 0
    println(if (ch.isLowerCase()) ch else "None")
}
