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

Example Input: cUCQXEFSgRZjK, RYdvUQXEFSgRZVlwx
Example Output: QXEFSgRZ

Example Input: nWTnWqkLqYTBxT, cSTnWqkLqYwXmxRF
Example Output: TnWqkLqY

Example Input: tWMLtmBGhcMlvjKpMNk, ihxBGhcMlvjKHJQDopB
Example Output:
BGhcMlvjK