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

Example Input: FgQsxN, UMQsIa
Example Output: Qs

Example Input: FCKbAgSVgj, wAEAWbAgSVSVwD
Example Output: bAgSV

Example Input: wnQgwbv, BsDQgwQr
Example Output:
Qgw