CF335B.Palindrome

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.

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

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.

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.