instruction:
In this task, you are given two strings A, B. Find the longest common substring in the strings A and B.
question:
HXYkGv, GCYksu
answer:
Yk


question:
wlFcxOHoXC, IWFcxOIWTk
answer:
FcxO


question:
ymdqZTBhocWDKhe, JoUvFNqZTBhoYIW
answer:

qZTBho