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

[EX Q]: wlFcxOHoXC, IWFcxOIWTk
[EX A]: FcxO

[EX Q]: RtOkkgLvhgfGlX, vCOkgLvhzOa
[EX A]: kgLvh

[EX Q]: LfYYSRBtw, QWYSRBAnw
[EX A]:
YSRB