using System;

using System.Collections.Generic;

public class Program

{

    public static void Main()

    {

    List<int> arr = new List<int> { 1, 2, 2, 4, 5, 2, 3, 4, 2, 5,};

    Console.WriteLine(TheBigN(arr));

    }

    public static bool TheBigN (List<int> arr) {

        for ( int i = 1; i < arr.Count; i++) 

        {

            if (arr[i-1] == arr[i] - 1 && arr[i-1] == arr[i + 1] - 2)

            {

                return true;

            }

            else

            {

                return false;

            }

        }

        return true;

    }   

}