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

Example Input: HKCoXQsLRtowYJ, PGRzNUQsLRtoLcqkn
Example Output: QsLRto

Example Input: XZslQqrShfCXRXyHrFVq, BtOQgQqrShfCXRRCRbMvXPM
Example Output: QqrShfCXR

Example Input: DqQmxE, FRQmMJ
Example Output:
Qm