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Time limit: 1 seconds

Memory limit: 256 megabytes

The bow whispers to the arrow before it speeds forth —‘Your freedom is mine.’ —Stray Birds, Poet, Rabindranath Tagore

题目描述

​ 初入PKU的晓神, 看见kcz等大神, 自认为除了竞赛方面要努力拿到XCPC金牌以外, 还需要为学术做出自己的贡献, 于是她打起了SCI一区论文的主意.

​ 可是作为北京大学计算机系大一本科生晓神, 在翻阅Science论文的时候, 看到众多晦涩学术词汇, 感到头昏眼花. 在那一刹那, 他看见了$N$个学术词语$S_{1\dots N}$, 由于长时间翻阅论文, 她似乎看见了这些这$N$个词汇中有$K$个不同的词汇组成的组合词.

​ Frank发现晓神在发呆, 为了掩饰自己发呆的晓神, 告诉Frank这$N$个学术词汇, 问Frank她看到的组合词到底是什么.

​ 特别地: 我们认为$S_i\neq S_j(i\neq j)$.

输入

第一行包含两个整数$N,K$.

接下来$N$行,每行包含一个字符串$S_i$.

输出

输出一个由$K$个$S_i$组成且字典序最小的字符串.

限制

• $1\leq K\leq N\leq50$.
• $1\leq|S_i|\leq50$.

5 2
z
z
zzz
z
zzzzzz

zz

2 2
b
ba

bab

2 1
b
ba

b

Postscript

For someone special: You can love someone and still choose to say goodbye to them. You can miss a person every day and still be glad they're no longer in your life.