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 OCaml (4.02.3) (judge verdict: Accepted)


let rec input n xs
    = if n = 0 then
          xs
      else
          let l = Scanf.scanf "%d " (fun l -> l) in
          input (n-1) (l :: xs)

let rec solve xs n res
    = if n <= 0 then
        Printf.printf "%d\n" res
      else
        solve xs (n-2) (res + (List.nth xs (n-1)))

let _ =
    let n = Scanf.scanf "%d " (fun n -> 2 * n) in
    let xs = input n [] in
    let xs = List.sort compare xs in
    let l = (List.length xs) - 1 in
    solve xs l 0