Excel spreadsheet with VBA code to create a temporary variable

I need to create an Excel spreadsheet that calculates the sum of two cells (A1 and B1) containing price data. The sum needs to be stored in a temporary variable and when either of the price values in A1 or B1 in the spreadsheet changes, the spreadsheet needs to compare the new sum with the old sum and only if the sum is different then it should write the prices to a new row in the spreadsheet.

For example:

Cell A1: 1.50
Cell B1: 2.50
Sum: 4.00

The spreadsheet must write to another worksheet tab a new row with the values:

1.50 2.50 4.00

Now I change Cell A1 to 2.00 and B1 stays the same. The new sum is 4.50

The spreadsheet must write to the other worksheet tab a new row with the values:

2.00 2.50 4.50

Now I change Cell B1 to 0.50 and A1 stays the same. The new sum is 2.50

The spreadsheet must write to the other worksheet tab a new row with the values:

2.00 0.50 2.50

Now I change Cell A1 to 8.00 and B1 to 0.75. The new sum is 8.75

The spreadsheet must write to the other worksheet tab a new row with the values:

8.00 0.75 8.75

Now I change Cell A1 to 8.10 and B1 to 0.65. The sum is 8.75

Since the new sum is the same as the old sum, the spreadsheet must not write a new row to the worksheet