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

Example Input: kOrQWjLKnwN, xFsWjLKgr
Example Output: WjLK

Example Input: XeQwOS, OuQwNR
Example Output: Qw

Example Input: POaYoKXbBsp, CIcMoKXbBNqVRm
Example Output:
oKXbB