using System;
using System.Linq;
using System.Text;

namespace StudyApp
{
    class Program
    {
        static void Main(string[] args)
        {
            string target = Console.ReadLine();
            StringBuilder sb = new StringBuilder();

            foreach (var c in target.Distinct())
                sb.Append(c);
            
            if (target.Equals(sb.ToString()))
                Console.WriteLine("YES");
            else
                Console.WriteLine("NO");
        }
    }
}