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

Example Input: STvKOHq, XMGvKOdyM
Example Output: vKO

Example Input: bmsrkXGb, GcarkXOZF
Example Output: rkX

Example Input: vrUDMwYlDQsGkJBUckdKv, THaMwYlDQsGkzagFRLRv
Example Output:
MwYlDQsGk