instruction:
In this task, you are given two strings A, B. Find the longest common substring in the strings A and B.
question:
ChKUlYUTQQBjZMZkgEed, lmQQBjZMZkgdEf
answer:
QQBjZMZkg


question:
JIOKyo, oPOKsX
answer:
OK


question:
vVvSfkTPrKBFMkkasttMI, DRqgPrKBFMkHxnnBCT
answer:

PrKBFMk