Problem Statement

We have N cards. The i-th card has a string S_i written on it.
Find the lexicographically smallest string that can be obtained by choosing K of these cards and concatenating them in any order.

Input

Input is given from Standard Input in the following format:
N K
S_1
S_2
$\vdots$
S_N

Output

Print the answer.

Constraints

• $1 \leq K \leq N \leq 50$
• $1 \leq |S_i| \leq 50$
• S_i consists of lowercase English letters.

Sample Input 1

4 3
ode
zaaa
r
atc

Sample Output 1

atcoder

Note that it is not possible to reverse or permute the string written on a card.
For example, ode written on the first card cannot be used as edo or deo.

Sample Input 2

5 2
z
z
zzz
z
zzzzzz

Sample Output 2

zz

There may be $a pair i, j (i\neq j)$ such that S_i = S_j.

见题面 Input 部分

见题面 Output 部分