instruction:
In this task, you are given two strings A, B. Find the longest common substring in the strings A and B.
question:
PQSFjlnU, kTSFjMUh
answer:
SFj


question:
FiKGCRXutEOzMqP, xdxRXutEsl
answer:
RXutE


question:
qqBgSa, WtBgBu
answer:

Bg