using System;
using static System.Console;
using System.Linq;
using System.Collections.Generic;

class Program
{
    static int NN => int.Parse(ReadLine());
    static int[] NList => ReadLine().Split().Select(int.Parse).ToArray();
    public static void Main()
    {
        Solve();
    }
    static void Solve()
    {
        var s = ReadLine();
        var count = new int[26];
        foreach (var c in s) ++count[c - 'A'];
        WriteLine(count.All(ci => ci < 2) ? "YES" : "NO");
    }
}