Excel 2007, 2010 Continuing subtraction?

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http://superuser.com – Tried everything! SUM, SUMPRODUCT, OFFSET, MATCH, INDIRECT, etc. etc. Just want to do what should be pretty basic for Excel: In A1 - 3000 or any value In A2 - another value In B1 (or any cell) return the sum of A1-A2, no problem with that , however, further values in A3, A4, A5 etc, i want Excel to subtract each one from the first value in A1 (at the time the value is input) and return me the answer in B1, (want the answer in the same cell) so really a continuing subtraction. Excel will do it really easily with addition but not subtraction. keep getting VALUE, REF errors all over the place. (HowTos)