using System;
using System.Linq;

public class P0406 {
    public static void Main() {
        var n = int.Parse(Console.ReadLine().Trim());
        var x = Console.ReadLine().Trim().Split(' ').Select(int.Parse).ToList();
        x.Sort();
        var result = x.SkipLast(1).Zip(x.Skip(1), (i, j) => j - i).Distinct().ToList();
        if(result.Count() == 1 && result[0] > 0) {
            Console.WriteLine("YES");
        } else {
            Console.WriteLine("NO");
        }
    }
}