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

namespace Application
{
	class MainClass
	{
		public static void Main (string[] args)
		{
			readInt ();
			var A = readInts (' ');

			var s = new Searcher ();
			var r = 0;
			var sortedA = A.OrderBy(ss => ss).Distinct().ToArray();
			while (r < sortedA.Length) {
				r += s.LonguestSeq (sortedA, r);
			}

			writeLine(s.Longuest);
		}

		public class Searcher
		{
			public int Longuest = 0;

			public int LonguestSeq (int[] A, int l)
			{
				var i = l + 1;
				for (; i < A.Count (); i++){
					if (A[i - 1] != A[i] - 1)
						break;
				}

				var len = i - l;

				this.Longuest = Math.Max(len, this.Longuest);

				return len;
			}
		}

		static void writeLine (object o)
		{
			System.Console.WriteLine (o.ToString ());
		}

		static string readItem ()
		{
			return readItems (' ') [0];
		}

		static String[] readItems (char c)
		{
			return System.Console.ReadLine ().Split (c);
		}

		static int readInt ()
		{
			return readInts (' ') [0];
		}

		static int[] readInts (char c)
		{
			return readItems (c).Select (x => int.Parse (x)).ToArray ();
		}
	}
}