In this task, you are given two strings A, B. Find the longest common substring in the strings A and B.

RtOkkgLvhgfGlX, vCOkgLvhzOa
kgLvh

RtAchtNymcswVk, FWRuRchtNymcswUkoERHyML
chtNymcsw

iyJrkHUm, FLJrkHBXHz

JrkH