In this task, you are given two strings A, B. Find the longest common substring in the strings A and B.

Example Input: xfZYYDKEqoRRu, ztCeYYDKEqoPG
Example Output: YYDKEqo

Example Input: rEHxRBOktYreNkK, NCOktYreFFCFwi
Example Output: OktYre

Example Input: PHHiSsY, PkVHiSQF
Example Output:
HiS