378QAQ has a string $s$ of length $n$. Define the core of a string as the substring$^\dagger$ with maximum lexicographic$^\ddagger$ order.

For example, the core of "$\mathtt{bazoka}$" is "$\mathtt{zoka}$", and the core of "$\mathtt{aaa}$" is "$\mathtt{aaa}$".

378QAQ wants to rearrange the string $s$ so that the core is lexicographically minimum. Find the lexicographically minimum possible core over all rearrangements of $s$.

$^\dagger$ A substring of string $s$ is a continuous segment of letters from $s$. For example, "$\mathtt{defor}$", "$\mathtt{code}$" and "$\mathtt{o}$" are all substrings of "$\mathtt{codeforces}$" while "$\mathtt{codes}$" and "$\mathtt{aaa}$" are not.

$^\ddagger$ A string $p$ is lexicographically smaller than a string $q$ if and only if one of the following holds:

• $p$ is a prefix of $q$, but $p \ne q$; or
• in the first position where $p$ and $q$ differ, the string $p$ has a smaller element than the corresponding element in $q$ (when compared by their ASCII code).

For example, "$\mathtt{code}$" and "$\mathtt{coda}$" are both lexicographically smaller than "$\mathtt{codeforces}$" while "$\mathtt{codeforceston}$" and "$\mathtt{z}$" are not.