// AtCoder My Practice
// author: Leonardone @ NEETSDKASU

[<EntryPoint>]
let main _ =
    let s =
        stdin.ReadLine().ToCharArray()
        |> Array.sort
    let n = Array.length s
    let xs = Array.skip 1 s
    let ys = Array.take (n-1) s
    let ans = Array.forall2 (=) xs ys
    if ans then
        printfn "yes"
    else
        printfn "no"
    0