Autonumbering from two columns

I have two different sets of columns (A:B and C:D. A and C contain the ID and B and D contain some data). They both run on the same series of ID (which is like a primary key in this case).

I would like that whenever there's a new entry in the column B or D, it will automatically give me the next ID in columns A or C, by checking both in column A and C for the latest ID which was used and add +1, thus generating the next available ID.

I attach excel as an example.Autonumbering.xlsx

Thanks!