instruction:
In this task, you are given two strings A, B. Find the longest common substring in the strings A and B.
question:
fVEFJa, WtEFYK
answer:
EF


question:
PQCjJwXurLeN, vsjJwXuOy
answer:
jJwXu


question:
hKqttOKfiw, gXqmttOKhTY
answer:

ttOK