Palindrome

Description:

Input:

The only line of the input contains one string s of length n (1 ≤ n ≤ 5·10⁴) 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.

Sample Input:

bbbabcbbb

Sample Output:

bbbcbbb

Sample Input:

rquwmzexectvnbanemsmdufrg

Sample Output:

rumenanemur

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.