object Main extends App{
  import scala.io.StdIn.readLine
  
  val s = readLine.split("").toList
  val delvaried = s.foldLeft(List(""))((a,b) => if(a.contains(b)){a}else a :+ b)
  
  if(s.length + 1 == delvaried.length)println("yes");
  else println("no")

}
