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

[EX Q]: bmsrkXGb, GcarkXOZF
[EX A]: rkX

[EX Q]: FgQsxN, UMQsIa
[EX A]: Qs

[EX Q]: dXkWzsrWKFspCfmbq, ZGXCnNrWKFspjaou
[EX A]:
rWKFsp