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

Example Input: pJqqIW, Ivqqxw
Example Output: qq

Example Input: HiGmCdXrmFRJYWLvZbpU, IxhkrmFRJYWLvZLaqRVxPkL
Example Output: rmFRJYWLvZ

Example Input: DYzieLsdDFH, XcieLsAgvV
Example Output:
ieLs