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.

Ex Input:
txxttppxpxtttxt

Ex Output:
xtttx


Ex Input:
vzzvlvvvvl

Ex Output:
lvvvvl


Ex Input:
tttkkkkhthkhk

Ex Output:

khthk