using System;

using System.Collections.Generic;

public class Program

{

    public static int CountDuplicates(int[] i){

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

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

        foreach(int x in i){

            if(!counting.Contains(x)) counting.Add(x);

            else if(!dupes.Contains(x)) dupes.Add(x);       

        }

        return dupes.Count;

    }

    public static void Main()

    {

        int[] input = {1,4,3,4,6,7,3,5,4,2,6,1};

        Console.WriteLine(CountDuplicates(input));

    }

}