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

Example Input: bmZKWkYPqugcvzbHykblPzQLhK, tPUzKqugcvzbHypzk
Example Output: qugcvzbHy

Example Input: zqwhMdAdXQfQCAmVNaYZm, toYMIwQeadAdXQfQCAmLyheFOv
Example Output: dAdXQfQCAm

Example Input: VjcupMGaIZmvBXANj, XWJSqmcupMGaIZmymkKWOdfh
Example Output:
cupMGaIZm