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

Example Input: gfDDsblwUryUw, UWaRisblwUTqTM
Example Output: sblwU

Example Input: iYuUgULgqcAgd, kRNUgULgqFv
Example Output: UgULgq

Example Input: JMLwPZvMixTFeTtwZZ, AcwPwPZvMixTdouHLK
Example Output:
wPZvMixT