Problem:
Score : 200 points

Problem Statement

Iroha has a sequence of N strings S_1, S_2, ..., S_N. The length of each string is L.

She will concatenate all of the strings in some order, to produce a long string.

Among all strings that she can produce in this way, find the lexicographically smallest one.

Here, a string s=s_1s_2s_3...s_n is lexicographically smaller than another string t=t_1t_2t_3...t_m if and only if one of the following holds:

There exists an index i(1≦i≦min(n,m)), such that s_j = t_j for all indices j(1≦j<i), and s_i<t_i.

s_i = t_i for all integers i(1≦i≦min(n,m)), and n<m.

Constraints

1 ≦ N, L ≦ 100

For each i, the length of S_i equals L.

For each i, S_i consists of lowercase letters.

Input

The input is given from Standard Input in the following format:

N L
S_1
S_2
:
S_N

Output

Print the lexicographically smallest string that Iroha can produce.

Sample Input 1

3 3
dxx
axx
cxx

Sample Output 1

axxcxxdxx

The following order should be used: axx, cxx, dxx.

Solution in C++ (GCC 9.2.1) (judge verdict: Accepted)

#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
#define rep(i,N) for(int i=0;i<int(N);++i)
#define rep1(i,N) for(int i=1;i<int(N);++i)
#define all(a) (a).begin(),(a).end()
#define bit(k) (1LL<<(k))
#define SUM(v) accumulate(all(v), 0LL)

typedef pair<int, int> i_i;
typedef pair<ll, ll> l_l;
template <class T> using vec = vector<T>;
template <class T> using vvec = vector<vec<T>>;
struct fast_ios{ fast_ios(){ cin.tie(0); ios::sync_with_stdio(false); cout << fixed << setprecision(20); }; }fast_ios_;

template<class T> inline bool chmax(T& a, T b) { if (a < b) { a = b; return true; } return false; }
template<class T> inline bool chmin(T& a, T b) { if (a > b) { a = b; return true; } return false; }

#define TOSTRING(x) string(#x)
template <typename T> istream &operator>>(istream &is, vector<T> &vec) { for (T &x : vec) is >> x; return is; }
template <typename T> ostream &operator<<(ostream &os, const vector<T> &v) { os  << "["; for(auto _: v) os << _ << ", "; os << "]"; return os; };
template <typename T> ostream &operator<<(ostream &os, set<T> &st) { os << "("; for(auto _: st) { os << _ << ", "; } os << ")";return os;}
template <typename T, typename U> ostream &operator<<(ostream &os, const pair< T, U >& p){os << "{" <<p.first << ", " << p.second << "}";return os; }
template <typename T, typename U> ostream &operator<<(ostream &os, const map<T, U> &mp){ os << "["; for(auto _: mp){ os << _ << ", "; } os << "]" << endl; return os; }

#define DUMPOUT cerr
void dump_func(){ DUMPOUT << endl; }
template <class Head, class... Tail> void dump_func(Head &&head, Tail &&... tail) { DUMPOUT << head; if (sizeof...(Tail) > 0) { DUMPOUT << ", "; } dump_func(std::move(tail)...); }

#ifdef DEBUG
#define dbg(...) dump_func(__VA_ARGS__)
#define dump(...) DUMPOUT << string(#__VA_ARGS__) << ": "; dump_func(__VA_ARGS__)
#else
#define dbg(...)
#define dump(...)
#endif

const int INF = (ll)1e9;
const ll INFLL = (ll)1e18+1;
const ll MOD = 1000000007;
// const ll MOD = 998244353;
const long double PI = acos(-1.0);

/*
const int dx[8] = {1, 0, -1, 0, 1, -1, -1, 1};
const int dy[8] = {0, 1, 0, -1, 1, 1, -1, -1};
const string dir = "DRUL";
*/


int main() {
    int N,L;
    cin >> N >> L;
    vector<string> S(N);
    cin >> S;
    sort(all(S));
    rep(i,N){
        cout << S[i];
    }
    cout << endl;
}