Circular reference help please!

Hi
I have a problem with circular reference in excel. I need a total
displayed in one cell address and i need it reduced accordingly by
entries into another cell address. eg- A3 contains 1000. A20 is used
for input and when operator puts in say 100 then A3 is reduced to 900.
We need up to 30 consecutive inputs. Any help would be appreciated -
Thanks

Are all cells A20:A49 to be subtracted from A3?

Where does the 1000 in A3 come from? Is that an input?

Hans