using System;
using System.Collections.Generic;
using System.Linq;

namespace Test
{
    class Program
    {
        static void Main(string[] args)
        {
            int n = int.Parse(Console.ReadLine());
            int[] x = Console.ReadLine().Split().Select(int.Parse).ToArray(); Array.Sort(x);
            int a = x[1] - x[0]; bool ret = a != 0;
            for (int i = 1; i < n; i++)
            {
                if (x[i] - x[i - 1] != a) ret = false;
            }
            Console.WriteLine(ret ? "YES" : "NO");
        }
    }
}