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

Example Input: xtCJGe, DXCJSw
Example Output: CJ

Example Input: VTwtMVKmrRvL, WLoVtWtMVKmrDCke
Example Output: tMVKmr

Example Input: ZyrpGdXvvtrKQIxU, CLNGdXvvtrKQDpicfqLdt
Example Output:
GdXvvtrKQ