adding or substracting number formulas

Help!!!! I've been trying to obtain the formula for adding to and subtracting from a original number such as in cell A1 the number is 100, the number in cell A2 is to be added to A1 and the number in A3 is to be subtracted from A1. Is there a formula to keep the number in A1?

eg. A1 - 100 then I add 50 in A2, now A1 is 150, A1 =IMSUB(A2,A3) is there a formula so that when I add or subtract (A3) that A1 automatically plus's or subtracts with out adding to A2 or subtracting from A3 to get the correct amount in A1?????

I'm using Excel 2010.

Thanks Really Confused