Excel How to extract two digits from Column A

I have a question about Excel How Can I extract two digits from Column B if Column B is empty extract two digits from column A. I know how to extract two digit from Column B but cannot understand how to do it if column B is empty.

for example: in column B in first row we have 123456 number we extract first 2 digits to column C, but in next row in column B is empty but column A is not empty and we need to extract number from column A.

Thanks for helping!

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Link to original article：https://stackoverflow.com/questions/73727158/excel-how-to-extract-two-digits-from-column-a