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 Java8 (OpenJDK 1.8.0) (judge verdict: Accepted)

import java.io.PrintWriter;
import java.util.Arrays;
import java.util.Scanner;

public class Main {
	static PrintWriter out = new PrintWriter(System.out);
	public static void main(String[] args) {
		// 入力
		final int n, l;
		final String[] s;
		try(Scanner scan = new Scanner(System.in)) {
			n = scan.nextInt();
			l = scan.nextInt();
			s = new String[n];
			for (int i = 0; i < n; i++) {
				s[i] = scan.next();
			}
			
		}
		Arrays.sort(s);
		
		// 出力
		Arrays.stream(s).forEach(out::print);
		out.println();
		out.flush();
	}

}