I have cell A1 and Cell A2.
A1=5
A2=no value yet.

I want to be able to type in a value in A2 that already has a formula which
subtracts A1 from the value I typed in.
Example,
A1=5. When I type 15 in A2, I want to see the result of 10 in A2(hence the
difference of A2-A1)