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

Example Input: RtOkkgLvhgfGlX, vCOkgLvhzOa
Example Output: kgLvh

Example Input: yOGvgtAmwMwcIRtJ, cQtAmwMwcIRjfjO
Example Output: tAmwMwcIR

Example Input: feYKRk, PrYKMW
Example Output:
YK