102255: [AtCoder]ABC225 F - String Cards

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.

Constraints

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

Input

Input is given from Standard Input in the following format:

$N$ $K$
$S_1$
$S_2$
$\vdots$
$S_N$

Output

Print the answer.

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 $S_i = S_j$.