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>
using namespace std;

typedef long long ll;
const ll INF = 1e9, MOD = 1e9 + 7;
const double EPS = 1e-9, PI = 3.141592653589793;
#define REP(i,a,b) for(ll i=a;i<b;i++)
#define rep(i,n) REP(i,0,n)
#define REPR(i,a,b) for(ll i=b;i>=a;i--)
#define repr(i,n) REPR(i,0,n)
#define All(v) (v).begin(), (v).end()
#define Pb push_back

int main(){
    cin.tie(0);
    ios::sync_with_stdio(false);

    ll n,l[201],ans=0;
    cin >> n;
    rep(i,2*n) cin >> l[i];
    sort(l,l+2*n,greater<int>());
    for(ll i=1;i<2*n;i+=2) ans += l[i];
    cout << ans << "\n";
    return 0;
}