/ 
Time Limit: 2 sec / Memory Limit: 128 MB
Problem Statement
Professor Y's work is to dig up ancient artifacts. Recently, he found a lot of strange stone plates, each of which has $N^2$ characters arranged in an $N \times N$ matrix. Further research revealed that each plate represents a message of length $N$. Also, the procedure to decode the message from a plate was turned out to be the following:
Select $N$ characters from the plate one by one so that any two characters are neither in the same row nor in the same column.
Create a string by concatenating the selected characters.
Among all possible strings obtained by the above steps, find the lexicographically smallest one. It is the message represented by this plate.
NOTE: The order of the characters is defined as the same as the order of their ASCII values (that is, $\mathtt{A} \lt \mathtt{B} \lt \cdots \lt \mathtt{Z} \lt \mathtt{a} \lt \mathtt{b} \lt \cdots \lt \mathtt{z}$).
After struggling with the plates, Professor Y gave up decoding messages by hand. You, a great programmer and Y's old friend, was asked for a help. Your task is to write a program to decode the messages hidden in the plates.
Input
The input is formated as follows:
$N$
$c_{11} c_{12} \cdots c_{1N}$
$c_{21} c_{22} \cdots c_{2N}$
:
:
$c_{N1} c_{N2} \cdots c_{NN}$
The first line contains an integer $N$ ($1 \le N \le 50$).
Each of the subsequent $N$ lines contains a string of $N$ characters.
Each character in the string is an uppercase or lowercase English letter (A-Z, a-z).
Output
Print the message represented by the plate in a line.
Sample Input 1
3 aab czc baa
Output for the Sample Input 1
aac
Sample Input 2
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 iiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiii
Output for the Sample Input 2
QQQQQQQQQQQQQQQQQQQQQQQQQiiiiiiiiiii
Sample Input 3
3 Acm aCm acM
Output for the Sample Input 3
ACM