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

Example Input: rcKVmDzwJKeK, uYqmDzwJKxkGi
Example Output: mDzwJK

Example Input: NwmTIZSnlKGG, qAYIZSnfWRd
Example Output: IZSn

Example Input: pWfvrm, bnfvAq
Example Output:
fv