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();
            var Count = 0;

            for (int i = 0; i < Input.Length; i++)
            {
                if (Input.Contains(Input[i])) Count++;
                if (Count == 2) { WriteLine("NO"); return; }
            }

            WriteLine("YES");
        }
    }
}