Problem:
Score : 200 points

Problem Statement

Snuke is having a barbeque party.

At the party, he will make N servings of Skewer Meal.

Example of a serving of Skewer Meal

He has a stock of 2N skewers, all of which will be used in Skewer Meal. The length of the i-th skewer is L_i.
Also, he has an infinite supply of ingredients.

To make a serving of Skewer Meal, he picks 2 skewers and threads ingredients onto those skewers.
Let the length of the shorter skewer be x, then the serving can hold the maximum of x ingredients.

What is the maximum total number of ingredients that his N servings of Skewer Meal can hold, if he uses the skewers optimally?

Constraints

1≦N≦100

1≦L_i≦100

For each i, L_i is an integer.

Input

The input is given from Standard Input in the following format:

N
L_1 L_2 ... L_{2N}

Output

Print the maximum total number of ingredients that Snuke's N servings of Skewer Meal can hold.

Sample Input 1

2
1 3 1 2

Sample Output 1

3

If he makes a serving using the first and third skewers, and another using the second and fourth skewers, each serving will hold 1 and 2 ingredients, for the total of 3.

Sample Input 2

5
100 1 2 3 14 15 58 58 58 29

Sample Output 2

135

Solution in C# (Mono 4.6.2.0) (judge verdict: Accepted)

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

class _C
{

	static int k(KeyValuePair<int,int>a,KeyValuePair<int,int>b)
	{
		return a.Key.CompareTo(b.Key);
	}

	void _()
	{
		var _n = _Ns();
		var N = _n[0];

		var L = _Ns();

		Array.Sort(L);
		int sum = 0;
		for (int i = 0; i < N*2; i += 2)
			sum+=L[i];


		Console.WriteLine(sum);			

	}

	static void Main(string[] args)
  {
    new _C()._();
  }

  #region

  string _S()
  {
    return Console.ReadLine();
  }

  string[] _Ss()
  {
    return Console.ReadLine().Split();
  }

  int[] _N(int n)
  {
    var x = new int[n];
    for (var i = 0; i < n; i++)
    {
      x[i] = int.Parse(Console.ReadLine());
    }
    return x;
  }

  int[] _Ns()
  {
    var ss = Console.ReadLine().Split();

    var x = new int[ss.Length];
    for (var i = 0; i < ss.Length; i++)
      x[i] = int.Parse(ss[i]);

    return x;
  }
  #endregion
}