import java.util.*

fun main(argv : Array<String>){
    
    val scan = Scanner(System.`in`)
    
    //
    val S = scan.next()
    var ca = S.toCharArray()
    Arrays.sort(ca)

    //
    var stop : Char = 'a'
    for(c in ca) {
        if(c==stop) {
            stop=c+1
        }
    }
    
    //
    if(stop=='z'+1)
        println("None")
    else
        println(stop)
    
}


