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

Example Input: pqFpUjQGiofBOjfYkRW, pQMnCFlhFpUjQGiofbc
Example Output: FpUjQGiof

Example Input: pYmzRnGPQVPrqPApKTyvaO, tNGPQVPrqPVTh
Example Output: GPQVPrqP

Example Input: PGMxIYmk, PWvxIYbc
Example Output:
xIY