instruction:
In this task, you are given two strings A, B. Find the longest common substring in the strings A and B.
question:
oCtWZYdkvZ, pkktWZYdEHMW
answer:
tWZYd


question:
kvutZfcPKzHE, kTKutZfcPTGmU
answer:
utZfcP


question:
amQQwn, GIQQFr
answer:

QQ