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++14 (GCC 5.4.1) (judge verdict: Accepted)

#include <bits/stdc++.h>
typedef long long ll;
typedef unsigned long long ull;
#define reps(i,s,n) for(ll (i)=(s);(i)<(n);++(i))
#define rep(i,n) reps(i,0,n)
#define reptr(i,n) for(ll (i)=(n);(i)>=0;--(i))
#define All(x) (x).begin(),(x).end()
#define Rall(a) (a).rbegin(), (a).rend()
#define vll vector<ll>
#define vi vector<int>
#define pll pair<ll,ll>
#define pi pair<int,int>
#define mp make_pair
#define pb push_back
#define fi first
#define se second
const ll MOD=1e9+7;
using namespace std;

int main(){
  int N;
  cin >> N;
  vector<int> l(2*N);
  rep(i,2*N){
    cin >> l[i];
  }
  sort(All(l));
  int ans = 0;
  for(int i=2*N-2;i>=0;i-=2){
    ans += l[i];
  }
  cout << ans << endl;
}