Modify Code that updates (adds/subtracts) One cell by the changing inputs of another

I would like to adapt a piece of code that I found and repurpose it.

Sample-2.xlsm

The code allows you to update cell ‘A1’ with continual changes in cell ‘A2’. (That is… put a number in A2, press enter and it modifies whatever is in A1 by that number) This works fine but I would like to be able to ‘add’ to cell A1 from cell A2 and ‘subtract’ from A1 from cell B2. The purpose of this is to avoid using negative numbers in user entry.

I would also like to be able, after update of cell A1 to clear cells A2 and B2. At present, clearing the last entry cancels the last change to A1. You can add the same entry to A2 twice then clear the second one but that is just looking for trouble.

Anyway, any help would be appreciated as always

Regards,
Scott