using System;

using System.Collections.Generic;

public class Program

{

    public static void Main()

    {

        int[] nums = {4,2,4,3,3,2,1};

        List<int> unic_el = new List<int>();

        for (int i = 0; i <= nums.Length - 1; i++)

        {

            if (!unic_el.Contains(nums[i]))

            {

                unic_el.Add(nums[i]);

            }

            else

            {

                unic_el.Remove(nums[i]);

            }

        }

        Console.WriteLine(unic_el[0]);

    }

}