using System;

using System.Linq;

public class Program

{

    public static void Main()

    {

        int[] arr = { 1, 55,55, 2, 3, 77,77, 1, 3, 2, 2, 4,4 ,4, 4};

​

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

            {

                if (arr[i] == arr[i + 1])

                {

                    Console.WriteLine(arr[i]);

                    return;

                }

            }

    }

}