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

namespace yukicoderTest
{
    class Program
    {
        static void Main(string[] args)
        {
            string input = Console.ReadLine();
            int index = 0;

            string temp = input.Substring(0, 2);
            index = input.IndexOf(temp, 2);
            if (index != -1)
            {
                temp = input.Substring(0, index);
                input = input.Replace(temp, "");
            }
            if(input=="")
            {
                Console.WriteLine("YES");
            }
            else
            {
                Console.WriteLine("NO");
            }
        }
    }
}