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)

    /*
    Life Before Death, Strength Before Weakness, Journey Before Destination
    ------------------------------------------------------------------------
    You don't become great by trying to be great.
    You become great by wanting to do something,
    and then doing it so hard that you become great in the process.
    */
 
    //#pragma comment(linker, ack:200000000")
    #pragma GCC optimize ("Ofast")
    #pragma GCC target ("sse,sse2,sse3,ssse3,sse4,popcnt,abm,mmx,avx,tune=native")
    #include <bits/stdc++.h>
    using namespace std;
    
    #define TRACE
    
    #ifdef TRACE
    #define trace(...) __f(#__VA_ARGS__, __VA_ARGS__)
    template <typename Arg1>
    void __f(const char* name, Arg1&& arg1){
      cerr << name << " : " << arg1 << std::endl;
    }
    template <typename Arg1, typename... Args>
    void __f(const char* names, Arg1&& arg1, Args&&... args){
      const char* comma = strchr(names + 1, ',');cerr.write(names, comma - names) << " : " << arg1<<" | ";__f(comma+1, args...);
    }
    #else
    #define trace(...)
    #endif
    
    #define rep(i, n)    for(ll i = 0; i < (n); ++i)
    #define repA(i, a, n)  for(ll i = a; i <= (n); ++i)
    #define repD(i, a, n)  for(ll i = a; i >= (n); --i)
    #define trav(a, x) for(auto& a : x)
    #define all(x) x.begin(), x.end()
    #define sz(x) (int)(x).size()
    #define fill(a)  memset(a, 0, sizeof (a))
    #define fst first
    #define snd second
    //#define mp make_pair
    #define pb push_back
    #define PI           3.141592653589793238462643383279502884197169399  /* pi */
    #define te  ll ti;   cin >> ti;  while(ti--) solve();  
    typedef long double ld;
    typedef long long ll;
    typedef pair<ll, ll> pii;
    typedef vector<ll> vi;
    const int maxN = 100100;

    vi v;

    void solve() {
        ll n, cnt = 0;
        cin >> n;
        n = 2*n;
        ll a[n];
        rep(i, n) cin >> a[i];
        sort(a, a+n);
        for(ll i=0; i<n; i+=2) cnt+=min(a[i], a[i+1]);
        cout << cnt << endl;

    }
 
    int main() {
      cin.sync_with_stdio(0); cin.tie(0);
      cin.exceptions(cin.failbit);
/*     
      #ifndef ONLINE_JUDGE
        freopen("input.txt", "r", stdin);
        freopen("output.txt", "w", stdout);
      #endif
*/     
      //te
      solve();
      return 0;
    }