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 $S_i = S_j$.

问题描述

我们有N张卡片。第i张卡片上写着一个字符串Si。

找出可以通过选择其中的K张卡片并按任意顺序连接它们得到的字典序最小的字符串。

限制条件

• 1≤K≤N≤50
• 1≤|Si|≤50
• Si由小写英文字母组成。

输入

输入格式如下：

N K
S1
S2
vdots
SN

输出

输出答案。

样例输入1

4 3
ode
zaaa
r
atc

样例输出1

atcoder

请注意，无法反转或排列卡片上写的字符串。
例如，第一张卡片上写的ode不能用作edo或deo。

样例输入2

5 2
z
z
zzz
z
zzzzzz

样例输出2

zz

可能存在一对i, j (i≠j)使得Si=Sj。