instruction:
In this task, you are given two strings A, B. Find the longest common substring in the strings A and B.
question:
AuSFmN, crSFXM
answer:
SF


question:
rvLoiycpZw, idcvoiycpcLyD
answer:
oiycp


question:
YXKhjC, CTKhvL
answer:

Kh