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

Example Input: feYKRk, PrYKMW
Example Output: YK

Example Input: XEvxzelZaWRO, zXIelZaWBUn
Example Output: elZaW

Example Input: erqWyeqUPKOFwYb, byXeqUPKOFwpAAws
Example Output:
eqUPKOFw