Match two excel column and output the matched value in C an D column

I have multiple excel files like this structure,I’d like to have the output highlighted in green,the logic is if B is in Column A then Output the value in column B; If B is not in Column A, then Output B.
I really not know how to achieve them,I’d appreciate if you could give me some sight.

Import Column A and Column B into datatables and compare them. You can either do that in the UiPath GUI,, or with C# LINQ,