using System;

using System.Collections.Generic;

public class Program

{

    public static void Main()

    {

        List<int> myarray = new List<int>() {34, 95, 34, 64, 45, 95, 16, 80, 80, 75, 3, 25, 75, 25, 31, 3, 64, 16, 31};

        lonelyinteger(myarray);

    }

    public static int lonelyinteger(List<int> a)

    {

        int lonelyBoy = 0;

        bool ansFound = false;

        a.Sort();

        for (int i=0; i<=a.Count-1; i+=2)

        {

            if (i != a.Count-1)

            {

                if (a[i] != a[i+1])

                {

                    lonelyBoy = a[i];

                    ansFound = true;

                    break;

                }

            }       

        }

        if (ansFound == false)

        {

            lonelyBoy = a[a.Count-1];

        }

        Console.WriteLine(lonelyBoy);

        return lonelyBoy;

    }

}