let id x = x
let s = Scanf.scanf "%s\n" id
let n = String.length s

let rec func c =
  if c = (n-1) then true else
    not( String.contains_from s (c+1) s.[c]) && func (c+1)
      
let ans = if func 0 then "yes" else "no"

let () =
  Printf.printf "%s\n" ans