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: vvwwviiiiwwv
Example Output: vwwv

Example Input: mnnnmmmmmnnmmm
Example Output: nnmmmmmnn

Example Input: knktnkktknntnn
Example Output:
nntnn