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, UiPath: How to compare two Data Tables - YouTube, or with C# LINQ, UiPath | Compare two Excel Sheets (/Data Tables) using C# LINQ | Tutorial - YouTube