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

Example Input: WFAPUCgVYbmZiFlacUzyv, xQFiekrgPUCgVYbmZUEtl
Example Output: PUCgVYbmZ

Example Input: cKsJcqYvZO, UrsJcqBC
Example Output: sJcq

Example Input: jdOXBRCuaGcFOplBNnq, MKOXWbCuaGcFOuurWI
Example Output:
CuaGcFO