using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace 競プロ1
{
    using static Console;

    class Program
    {
        static void Main(string[] args)
        {
            var Input = ReadLine();
            char[] Chosen;
            for (int i = 0; i < Input.Length;)
            {
                Chosen = Input.Where(s => Input[i] == s).ToArray();
                if (Chosen.Length > 1)
                {
                    WriteLine("NO");
                    return;
                }

                else {WriteLine("YES"); return; }
            }
        }
    }
}