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

Example Input: euQWhCovKplukMYznXcD, XAfMhKdzhCovKplukMcMlSuBw
Example Output: hCovKplukM

Example Input: HXYkGv, GCYksu
Example Output: Yk

Example Input: SnPoko, BvPoAu
Example Output:
Po