object Main extends App {
    val pre_s = io.StdIn.readLine

    val s = pre_s.distinct
    
    var result = "None"
    var has = false
    for (char <- "abcdefghijklmnopqrstuvwxyz" if !has) {
        if (! s.contains(char)) {
            has = true
            result = char.toString
        }
    }

    println(result)
}