# Problem B

Repeated Subsequence

You are given a string $s$. Find the **longest** string $t$ such that $t$ appears as a subsequence of
$s$ at least twice.

A string $t$ is a subsequence of $s$ if you can highlight some characters of $s$ such that, when read in order from left to right, they form the string $t$. For example, $aca$ is a substring of $abeacbad$, because we can highlight indices $4$, $5$, and $7$, as shown below.

We say a string $t$
appears as a subsequence at least twice if there are two
different sets of indices of $s$ that can be highlighted to form
$t$. Two sets are
considered different if there is **at
least** one position that is included in one set but not the
other. For example, $aca$
appears as a subsequence of $abeacbad$ at least twice because we
can highlight either of the below sets of indices (note that it
is not the longest such subsequence):

## Input

The first line of the input contains a single integer $t$ ($1 \le t \le 10^4$) —the number of test cases. The description of the test cases follows.

The first line of each test case contains a single integer $n$ ($1 \le n \le 2\cdot 10^5$) —the size of the string $s$.

The second line of each test case contains $n$ lowercase letters —the string $s$.

It is guaranteed that the sum of $n$ over all test cases does not exceed $2\cdot 10^5$.

## Output

For each test case, if no subsequence of $s$ appears multiple times, print $-1$.

Otherwise, output the longest string $t$ that appears as a subsequence of $s$ multiple times.

If there are multiple solutions, print any.

Sample Input 1 | Sample Output 1 |
---|---|

7 1 h 3 aab 3 xyz 7 ababbab 9 abceyzvpa 8 repeated 11 subsequence |
-1 ab -1 ababab a reated subseque |