using System;

using System.Collections.Generic;

​

namespace ArrayPairSum

{

    public class Program

    {

       public static void Main(string[] args)

        {

            int[] array = new int[] { 2, 4, 5, 7, 8, 9, 3, 6 };

            int sum = 10;

            int result = GetPairCount(array, sum);

            Console.WriteLine("Number of pairs: " + result);

        }

​

        static int GetPairCount(int[] array, int sum)

        {

            int count = 0;

            HashSet<int> set = new HashSet<int>();

            foreach (int num in array)

            {

                //Console.WriteLine(num);

                int target = sum - num;

                Console.WriteLine(target);

                if (set.Contains(target))

                {

                    Console.WriteLine(set);

                    count++;

                }

                set.Add(num);

            }

            return count;

        }

    }

}