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 LuaJIT (2.0.4) (judge verdict: Accepted)

local read = io.read
local byte = string.byte
local N, L = read("*n", "*n")
read("*l")
local S = {}
for i = 1, N do
	local str = read("*l")
	S[i] = {byte(str, 1, L)}
	setmetatable(S[i], {__call = function() return str end})
end

local function isDict(s1, s2)
	for i = 1, L do
		if s1[i] < s2[i] then
			return true
		elseif s1[i] > s2[i] then
			return false
		end
	end
	return false
end

table.sort(S, isDict)
local out = ""
for i = 1, N do
	out = out..S[i]()
end

print(out)