using System;

using System.Collections.Generic;

using System.Linq;

public class Program

{

    public static void Main()

    {

        int[] tab = { 1, 2, 3, 2, 1 };

        Console.WriteLine(GetSingle(tab));

    }

    private static int GetSingle(IEnumerable<int> tab)

    {

        return tab.Aggregate(0, (current, i) => current ^ i);

    }

}