instruction:
In this task, you are given two strings A, B. Find the longest common substring in the strings A and B.
question:
RtOkkgLvhgfGlX, vCOkgLvhzOa
answer:
kgLvh


question:
MriFNpGM, yaiFNduk
answer:
iFN


question:
SdTsifRBrXoNrebtT, iZofRBrXoNdJ
answer:

fRBrXoN