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

Example Input: UZbmGVDzIaf, qfHwqGVDzIsd
Example Output: GVDzI

Example Input: IGHhMfqLeBpnullsPInfBH, LTDBpnullsPIBynwCrCQ
Example Output: BpnullsPI

Example Input: VWWCkirOkvisH, DdSCkirOkvijdTsButU
Example Output:
CkirOkvi