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;

#define ll long long
#define mp make_pair
#define pb push_back
#define pii pair <int, int>
#define piii pair<int, pii>
#define vi vector<int>
#define vpii vector<pii>

#define read1(a) int a; scanf("%d", &a)
#define read2(a, b) int a, b; scanf("%d %d", &a, &b)
#define read3(a, b, c) int a, b, c; scanf("%d %d %d", &a, &b, &c)

#define FOR(i, a, b) for (int i=a; i<b; i++)
#define F0R(i, a) for (int i=0; i<a; i++)

#define readgi(n) F0R(i, n) { scanf("%d", &arr[i]); }
#define readgs(n) F0R(i, n) { scanf(" %c", &arr[i]); }

#define f first
#define s second

#define usaco(in, out) freopen(in, "r", stdin); freopen(out, "w", stdout);

#define println1(a) printf("%d\n", a);
#define println2(a, b) printf("%d %d\n", a, b);
#define println3(a, b, c) printf("%d %d %d\n", a, b, c);
#define pv(v) for (int i : v) { printf("%d ", i); } printf("\n");

const int MOD = 1000000007;
const int MAX = 250005;

int arr[MAX];

int main() {
    read1(n);
    F0R(i, 2*n) { scanf("%d", &arr[i]); }
    sort(arr, arr+2*n);
    ll tot = 0;
    F0R(i, n) {
        tot += arr[2*i];
    }
    cout << tot << endl;
}