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 <iostream>
#include <string>
#include <cstdlib>
#include <cstdio>
#include <queue>
#include <stack>
#include <list>
#include <utility>
#include <vector>
#include <set>
#include <algorithm>
#include <cmath>
#include <climits>
#include <map>
#include <unordered_map>
#include <string.h>
#include<iomanip>
#include <iterator>
#include <locale>

#define all(a) a.begin(),a.end()
#define VV(T) vector<vector< T > >
#define rep(i,N) for(int i = 0; i <N; i++)
#define repsg(i,s,g) for(int i = s; i <g; i++)
#define name(x) #x
using namespace std;

const int INF = INT_MAX;
const long long INFL =LONG_LONG_MAX;
const double PI = atan(1)*4;
const int MOD = pow(10,9)+7;
int dy[]={0, 0, 1, -1, 0};
int dx[]={1, -1, 0, 0, 0};

typedef long long ll,LL;
typedef vector<int> vint;
typedef vector<vector<int> > vvint;
typedef vector<long long> vll, vLL;
typedef vector<vector<long long> > vvll, vvLL;
typedef pair<int,int> pii;
typedef pair<LL, LL> pLL;


// vector入力
template<typename T>
istream& operator >> (istream& is, vector<T>& vec){
    for(T& x: vec) is >> x;
    return is;
}
// vector出力
template<typename T>
ostream& operator << (ostream& os, vector<T>& vec){
    for(int i=0; i<vec.size(); i++){
        os << vec[i] << ( i+1 == vec.size() ? "" : " " );
    }
    return os;
}


int main() {
    int n;cin>>n;
    vector<int>L(2*n);cin>>L;
    sort(all(L));
    int ans = 0;
    rep(i,n){
        ans += L[2*i];
    }
    cout<<ans<<endl;
    
    
    return 0;
}