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

Example Input: pYmzRnGPQVPrqPApKTyvaO, tNGPQVPrqPVTh
Example Output: GPQVPrqP

Example Input: XZslQqrShfCXRXyHrFVq, BtOQgQqrShfCXRRCRbMvXPM
Example Output: QqrShfCXR

Example Input: BrRKVMCCqsMUDWbvvu, khDUCqsMUDWkXBR
Example Output:
CqsMUDW

