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

Example Input: CkZOSZevST, bbZOSZIg
Example Output: ZOSZ

Example Input: zBxuZfJgYrlRSzkmAktO, YNMgYrlRSzklmBOH
Example Output: gYrlRSzk

Example Input: JFQXBgmQImP, DzLQXBgmNp
Example Output:
QXBgm