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 Haskell (GHC 7.10.3) (judge verdict: Accepted)

module Main where
    import Data.List
    import Control.Monad
    
    main = do
        getLine
        inputs<-getInputs
        let needle =  reverse $ sort inputs
        print $ foldr (+) 0 (get2nds needle)

    get2nds::[a]->[a]
    get2nds [] =[]
    get2nds [x] =[x]
    get2nds (x:[y]) = [y]
    get2nds (x:y:xs)= y:(get2nds xs)
    
        
    read' s = read s ::Integer

    getInputs = do
        line <- getLine
        let inputs = map read' (words line)
        return inputs

    getLines n = do
        lines <- (replicateM n getLine)
        return lines

    getGCD::Integer->Integer->Integer
    getGCD m n 
        | m < n = (getGCD n m)
        | n == 0 = m
        | otherwise = getGCD (mod m n) n

    getLCM::Integer->Integer->Integer
    getLCM m n = div (m * n) (getGCD m n)