using System;

public class Program

{

    public static void Main()

    {

        int[] Socks = {1,1,2,7,10,5,7,10,5,9,9,9};

        int n = Socks.Length;

        Array.Sort<int>(Socks);

        int pair = 0;

        for(int i=0; i<=n-2;i++)

        {

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

                {   Console.WriteLine(Socks[i]);

                    pair++;

                    i++;

                }

        }

        Console.WriteLine(pair);

    }

}