import java.util.*

val sc = Scanner(System.`in`)
val S: String = sc.next()

fun main(args: Array<String>) {
    println(if (judge(S)) "yes" else "no")
}

fun judge(s: String): Boolean {
    val dic = mutableSetOf<Char>()
    for (c in s) {
        if (c in dic) {
            return false
        }
        dic.add(c)
    }
    return true
}
