using System;

using System.Collections.Generic;

public class Program

{

    public static void Main()

    {

        Console.WriteLine("Hello World");

        int[] a={1,2,3,4,2,1,5,-1};

        Dictionary<int,int> checker =new Dictionary<int,int>();

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

        {

            if(checker.ContainsKey(a[i]))

               {

                   int count = checker[a[i]];

                   count++;

                   checker[a[i]] =count;

               }

               else

               {

                    checker.Add(a[i],1);

               }

        }

        foreach (KeyValuePair<int, int> pair in checker)

        {

        Console.WriteLine("{0}, {1}",

        pair.Key,

        pair.Value);

        }

    }

}