import java.util.Scanner
import java.util.HashSet

fun main(args: Array<String>) {
  val cin = Scanner(System.`in`)
  val str = cin.next()

  var hsc: HashSet<Char> = hashSetOf()

  for (c in str) {
    hsc.add(c)
  }

  if (str.length == hsc.size) {
    println("yes")
  } else {
    println("no")
  }
  cin.close()
}