using System;

public class Program

{

    public static int LSS=0;

    public void LS(int[] arr)

    {   

        //Array.Sort(arr);

        for(int i=0;i<arr.Length;i++)

        {

            check(arr,i+1,1,arr[i]);    

        }

    }

    public void check(int[] arr,int start,int count,int number)

    {

        if(count>LSS)

        {

            LSS=count;  

        }

        if(start==arr.Length)

        {

                return;

        }

        else {

            for(int i=start;i<arr.Length;i++)

            {               

                if(arr[i]-number==0 || arr[i]-number==1)

                {       

                    check(arr,i+1,count+1,arr[i]);

                }   

                else if(number-arr[i]==0 || number-arr[i]==1)

                {

                    check(arr,i+1,count+1,arr[i]);

                }

            }

        }

    }

    public static void Main()

    {

        int[] arr=new int[]{1, 2, 3, 2, 3, 7, 2, 1};

        Program program=new Program();

        program.LS(arr);

        Console.WriteLine(LSS);

    }

}