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 OCaml (4.02.3) (judge verdict: Runtime Error)

(* let sl = Str.split (Str.regexp " ") @@ read_line () *)
(* let il = List.map int_of_string sl *)
(* let ar = Array.of_list il *)

(* let (a, b) = Scanf.scanf "%d %d\n" (fun a b -> a, b) *)
(* let () = Printf.printf "%d %f\n" 2 3.5 *)




let (n, l) = Scanf.scanf "%d %d\n" (fun a b -> a, b)
let s = Array.make n ""

let () =
  for i = 0 to n-1 do
    s.(i) <- read_line ()
  done

let () =
  Array.sort compare s;
  for i = 0 to n-1 do
    print_string s.(i)
  done;
  print_newline ()