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

Example Input: YXsgPE, ENsgSO
Example Output: sg

Example Input: BrDGrTD, PBHDGrDw
Example Output: DGr

Example Input: dWQBMqLeCksksbRHWVtbgWE, ddLeCksksbRZZYf
Example Output:
LeCksksbR

