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

Example Input: bmsrkXGb, GcarkXOZF
Example Output: rkX

Example Input: GFhJafTV, waHhJaji
Example Output: hJa

Example Input: peWIPNiJqORiWAAqNknR, cidztPNiJqORiWCuR
Example Output:
PNiJqORiW