instruction:
In this task, you are given two strings A, B. Find the longest common substring in the strings A and B.
question:
lrBJDRkT, jmBJDrI
answer:
BJD


question:
RMurrwPjlfG, QBKrrwPDkTM
answer:
rrwP


question:
DgFsSvHKusqztQub, PRNYPFsSvHKusqyecqpP
answer:

FsSvHKusq