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문제

Ahmed El-Sayed, the ACPC Regional System Administrator, has generated passwords for each team to login to the pc^2 program used to run the contest. Unfortunately for him, he lost these passwords! But fortunately for us, he remembered how he generated them.

To generate the passwords, Ahmed has a list X of N strings. He defines a list P as a permutation of list X if and only if P and X contain the same elements but not necessarily in the same order. He then generates all possible permutations of X. He then creates a new list Y such that it contains a string S for each permutation P of X, S is the concatenation of all the strings in the permutated list P. Finally, he sorts the list Y in lexicographic order (according to the ASCII table) and removes duplicate strings from Y. Then the passwords are the strings at some predefined indices in Y.

Given index K (starting from 1) and the initial list of strings X, you should tell Ahmed the password in index K.

입력

The first line of input contains an integer T, the number of test cases. Each test case consists of two integers N ≤ 20 (size of list X) and K (always a valid value) followed by N strings. Each string consists of maximum 10 characters and all strings will be of equal size. The strings have no whitespaces and consist only of alphabetical characters.

 

출력

For each test case, on a separate line, print the Kth password as explained above.

예제 입력

1
3 3
w
y
d

예제 출력

wdy

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