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 PHP7 (7.0.15) (judge verdict: Wrong Answer)

<?php
  $N = 0;
  $L = 0;
  $strArray = [];
  $count = 0;
  while ( ! feof(STDIN) ) {
    $line = trim(fgets(STDIN));
    if ($count === 0) {
      $tmp = explode(" ", $line);
      $N = $tmp[0];
      $L = $tmp[1];
    } else {
      $strArray[] = $line;
    }
    $count++;
  }
  sort($strArray);
  $str = "";
  for ($i = 0; $i < $L; $i++) {
    $str .= $strArray[$i];
  }
  echo $str;