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

Example Input: ZhHtTXaUmsQfUMNrnt, KvvebSXaUmsQfBspwM
Example Output: XaUmsQf

Example Input: inquDFuJxtBaaFGCaRzO, NYcnWDuDFuJxtBafxZWCUw
Example Output: uDFuJxtBa

Example Input: AuSFmN, crSFXM
Example Output:
SF