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

Example Input: QvtDqzQiSknPbRCG, hVxHdqzQiSknPbumdpNIf
Example Output: qzQiSknPb

Example Input: GrvtMjEvRlmN, ifGOWMjEvRPSOZK
Example Output: MjEvR

Example Input: etxZbJ, BExZBe
Example Output:
xZ