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

namespace ConsoleApp1
{
    class Program
    {
        static void Main(string[] args)
        {
            var input = Console.ReadLine();
            var len = input.Length;
            var result = false;
            for (int i = 0; i < len; i++)
            {
                var aa = input[i];
                for (int j = i+1; j < len; j++)
                {
                    if (aa != input[j]) continue;
                    result = true;
                    break;
                }

                if (result)
                {
                    break;
                }
            }

            Console.WriteLine(result ? "NO" : "YES");
        }
    }
}