In this task you will be given a string and you should find the longest substring that is a palindrome. A palindrome is a string that is the same backwards as it is forwards. If the shortest possible palindrome is length 1 you should return the first character.

Example Input: xxmxxmmmmmm
Example Output: mmmmmm

Example Input: zzzzyyyyzyzzzz
Example Output: zyyyyz

Example Input: nkkssnsnkkn
Example Output:
nkkn