instruction:
In this task, you are given two strings A, B. Find the longest common substring in the strings A and B.
question:
RMurrwPjlfG, QBKrrwPDkTM
answer:
rrwP


question:
BfCCzPNLBbwCoxMVAG, vhwNLBbwCbH
answer:
NLBbwC


question:
UMnKBy, WTnKFn
answer:

nK