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: pppptttptpppp
Example Output: ptttp

Example Input: gcgmmmmcgccmm
Example Output: mmmm

Example Input: zzzzzikkikizkk
Example Output:
zzzzz