using System;
using System.Collections.Generic;


namespace rummy
{
    class Program
    {
        /// <summary>
        /// プログラムのエントリポイント
        /// </summary>
        /// <param name="args"></param>
        static void Main(string[] args)
        {
            Console.ReadLine();

            string[] tmp = Console.ReadLine().Split(' ');

            List<int> list = new List<int>();
            foreach (string s in tmp)
                list.Add(int.Parse(s));

            list.Sort();

            Console.WriteLine(JudgeInterval(list));
        }

        private static string JudgeInterval(List<int> list)
        {
            for (int i = 0; i < list.Count - 1; i++)
                if (list[i] == list[i + 1])
                    return "NO";

            int interval = list[0] - list[1];
            for (int i = 1; i < list.Count - 1; i++)
                if (list[i] - list[i + 1] != interval)
                    return "NO";

            return "YES";
        }
    }
}