using System;

using System.Linq;

public class Program

{

    public static void Main()

    {       

        //https://stackoverflow.com/questions/69243543/

            int[] num;

            int accum = 0;

            string input = "1,2,3,1,2";

​

            num = input.Split(',').Select(int.Parse).ToArray();

​

​

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

                accum ^= num[i];

            Console.WriteLine(accum);

    }

 }