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

class Program
{
    static void Main(string[] args)
    {
        int n = int.Parse(Console.ReadLine());
        int[] x = Console.ReadLine().Split().Select(v => int.Parse(v)).ToArray();

        Array.Sort(x);

        if (Judge(x))
        {
            Console.WriteLine("YES");
        }
        else
        {
            Console.WriteLine("NO");
        }

        Console.Read();
    }

    static bool Judge(int[] x)
    {
        int n = x[1] - x[0];

        if (n == 0)
        {
            return false;
        }

        for (int i = 2; i < x.Length - 1; i++)
        {
            if (x[i + 1] - x[i] != n)
            {
                return false;
            }
        }

        return true;
    }
}