Easy

23.7%

Given a string **str**, find the longest repeating non-overlapping substring in it. In other words find 2 identical substrings of maximum length which do not overlap. If there exists more than one such substring return any of them.

**Input:**

The first line of input contains an integer T denoting the number of test cases. Then T test cases follow. The first line of each test case contains an integer N denoting the length of the string str.

The second line of each test case contains the string str.It consists of only lower case english alphabets.

**Output:**

Print the longest non - overlapping substring for each test case in a new line. If no such sub - sequence exists print -1.

**Constraints:**

1<= T<=100

1<= N <=1000

**Example:**

Input:

1

13

geeksforgeeks

Output:

geeks