instruction:
In this task, you are given two strings A, B. Find the longest common substring in the strings A and B.
question:
feYKRk, PrYKMW
answer:
YK


question:
wbdHVigIkjNkuAmbr, iSgIkjNkQYyn
answer:
gIkjNk


question:
oIEGCQltCXcN, VfgkCQltCZkXj
answer:

CQltC