instruction:
In this task, you are given two strings A, B. Find the longest common substring in the strings A and B.
question:
yiKkIxbKvNk, znaIxbKggg
answer:
IxbK


question:
PQSFjlnU, kTSFjMUh
answer:
SFj


question:
jfHBWB, VKHBbC
answer:

HB