题目描述

Y教授的工作是挖掘古代文物。最近，他发现了许多奇怪的石板，每块石板上排列着$N^2$个字符，组成一个$N \times N$的矩阵。进一步的研究揭示出每块石板代表着一个长度为$N$的消息。解码这些消息的步骤如下：

1. 逐个从石板上选择$N$个字符，使得任意两个字符都不在同一行也不在同一列。
2. 将所选择的字符连接起来，得到一个字符串。
3. 在上述步骤得到的所有可能字符串中，找到字典序最小的字符串，即为该石板代表的消息。

注意：字符的顺序按照它们的ASCII值的顺序确定（即$\mathtt{A} < \mathtt{B} < \cdots < \mathtt{Z} < \mathtt{a} < \mathtt{b} < \cdots < \mathtt{z}$）。

在研究了这些石板后，Y教授放弃了手工解码消息的尝试。而你作为一位优秀的程序员和Y教授的老朋友，被要求帮助他。你的任务是编写一个程序来解码这些隐藏在石板中的消息。

输入

输入的格式如下：

$N$
$c\_{11} c\_{12} \cdots c\_{1N}$
$c\_{21} c\_{22} \cdots c\_{2N}$
:
:
$c\_{N1} c\_{N2} \cdots c\_{NN}$

第一行包含一个整数$N$ $(1 \le N \le 50)$。接下来的$N$行，每行包含长度为$N$的字符串。每个字符串中的字符都是大写或小写英文字母（A-Z，a-z）。

输出

输出石板所代表的消息。

示例输入1

3
aab
czc
baa```

### 示例输出1

```plain
aac```

---

### 示例输入2

```plain
36
iiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiii
iiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiii
iiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiii
iiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiii
iiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiii
iiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiii
iiiiiiiiiiQiiiiiiiiiiiiiiiQiiiiiiiii
iiiiiiiiiiQQQiiiiiiiiiiQQQQiiiiiiiii
iiiiiiiiiiQQQQQiiiiiiiQQQQiiiiiiiiii
iiiiiiiiiiiQQQQQQQQQQQQQQQiiiiiiiiii
iiiiiiiiiiiQQQQQQQQQQQQQQQiiiiiiiiii
iiiiiiiiiiiQQQQQQQQQQQQQQiiiiiiiiiii
iiiiiiiiiiiiQQQQQQQQQQQQQiiiiiiiiiii
iiiiiiiiiiiQQQQQQQQQQQQQQQiiiiiiiiii
iiiiiiiiiiQQQQQQQQQQQQQQQQQiiiiiiiii
iiiiiiQiiiQQQQQQQQQQQQQQQQQiiiQiiiii
iiiiiiQQiQQQQQQQQQQQQQQQQQQiiQQiiiii
iiiiiiiQQQQQQQQQQQQQQQQQQQQiQQiiiiii
iiiiiiiiiQQQQQQQQQQQQQQQQQQQQiiiiiii
iiiiiiiiQQQQQiiQQQQQQQQiiQQQQQiiiiii
iiiiiiQQQQQQiiiiQQQQQiiiiQQQQQQiiiii
iiiiiQQQQQQQQiiiQQQQQiiQQQQQQQQiQiii
iiiQQQQQQQiiQiiiQQQQQiiQiiQQQQQQQQii
iQQQQQQQQQiiiiiQQQQQQQiiiiiiQQQQQQQi
iiQQQQQQQiiiiiiQQQQQQQiiiiiiiiQQQiii
iQQQQiiiiiiiiiQQQQQQQQQiiiiiiiiQQQii
iiiiiiiiiiiiiiQQQQQQQQQiiiiiiiiiiiii
iiiiiiiiiiiiiQQQQQQQQQQiiiiiiiiiiiii
iiiiiiiiiiiiiQQQQQQQQQQiiiiiiiiiiiii
iiiiiiiiiiiiiiQQQQQQQQiiiiiiiiiiiiii
iiiiiiiiiiiiiiQQQQQQQQiiiiiiiiiiiiii
iiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiii
iiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiii
iiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiii
iiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiii```

### 示例输出2

```plain
QQQQQQQQQQQQQQQQQQQQQQQQQiiiiiiiiiii```

---

### 示例输入3

```plain
3
Acm
aCm
acM```

### 示例输出3

```plain
ACM```

---

### 来源

[Japan Alumni Group Spring Contest 2014](http://acm-icpc.aitea.net/index.php?2013%2FPractice%2F%BD%D5%A5%B3%A5%F3%A5%C6%A5%B9%A5%C8%2F%B0%C6%C6%E2)

---