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

Example Input: yOGvgtAmwMwcIRtJ, cQtAmwMwcIRjfjO
Example Output: tAmwMwcIR

Example Input: YHLJZrFQ, aSLJZQuo
Example Output: LJZ

Example Input: RWsAyFtDnzoL, HwsAyFteh
Example Output:
sAyFt