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

Example Input: SeXNqRAbKdLuWXgb, npsIWRAbKdLuUWSbvuS
Example Output: RAbKdLu

Example Input: CkZOSZevST, bbZOSZIg
Example Output: ZOSZ

Example Input: FIssNf, PxssMb
Example Output:
ss