How to add the correct number in column A.

Hello,

I need to do an import for work but I can find the way to make the information compatible with he import tool.
This is only one of the files, and manually it would take to much time.

Every time if in column B the number changes, Column A has to take that number and place it on the same line.

In attachment I have a folder that shows the expected result, its hard for me to explain via text.

Is there an easy way to do this?

Best regards.