using System;
using System.Linq;

public class Program
{
	public static void Main()
	{
		int n = int.Parse(Console.ReadLine());
		int[] ducks = Console.ReadLine().Split().Select(i => int.Parse(i))
			.OrderBy(i => i).ToArray();
		int tolerance = ducks[1] - ducks[0];
		if (tolerance == 0)
		{
			Console.WriteLine("NO");
			return;
		}
		for (int i = 2; i < n; i++)
		{
			if (ducks[i] - ducks[i - 1] != tolerance)
			{
				Console.WriteLine("NO");
				return;
			}
		}
		Console.WriteLine("YES");
	}
}