Given a string $*s*$, determine if it contains any palindrome of length exactly $100$ as a subsequence. If it has any, print any one of them. If it doesn't have any, print a palindrome that is a subsequence of $*s*$ and is as long as possible.

Input

The only line of the input contains one string $*s*$ of length $*n*$ ($1 ≤ *n* ≤ 5·10^{4}$) containing only lowercase English letters.

Output

If $*s*$ contains a palindrome of length exactly $100$ as a subsequence, print any palindrome of length $100$ which is a subsequence of $*s*$. If $*s*$ doesn't contain any palindromes of length exactly $100$, print a palindrome that is a subsequence of $*s*$ and is as long as possible.

If there exists multiple answers, you are allowed to print any of them.

Note

A subsequence of a string is a string that can be derived from it by deleting some characters without changing the order of the remaining characters. A palindrome is a string that reads the same forward or backward.