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

Example Input: IKrQfT, crrQRg
Example Output: rQ

Example Input: vSFxjd, AHFxpi
Example Output: Fx

Example Input: rRdgecoqemB, BpktngecoqeciUNW
Example Output:
gecoqe