On a worksheet:

In cell A1 is a number.
In cell B1 is another number.
In cell C1 is the formula: = sum(A1+B1)

The values in A1 and B1 are downloaded from the web and change constantly.

What I want to do is to use cell D1 to accumulate the values of C1.

For example
A1 = 100, B1 = 200, C1 = 300, D1 = 300
A1 = 200, B1 = 300, C1 = 500, D1 = 300 + 500 = 800, etc.

Presumably, I can use to a worksheet change event to capture the changes in
C1 to put them in D1. The problem is C1 depends on A1 and B1. Thus, when
there is a new value in A1, C1 changes. Then when there's a new value in B1,
C1 changes again. So the value in A1 will be double counted. How can I make
sure that the worksheet change event only fires when both A1 and B1 have new
values?

TIA.