Adding up in Excel.

Hi.
First of all, thanks again for this fantastic Forum. I would like to know if this is possible to do.
Cell A1 is currently empty and without any formulas in it. I am going to enter data in B1. Lets say 100. As soon as I press enter this value must show in cell A1. Here is the trick. As soon as this value is entered in A1, I want B1 to go blank again. Now I enter 200 in B1 and after entering A1 must show 300, (100+200) and B1 must go blank again.

The reason for this is I have to update the worksheet every day with a lot of data and at the moment I had to add the data on the sheet to the data I want to update with using a calculator and then enter it back in A1. If there is a formula for this I can update A1 everytime by just putting the new value I want to add in B1 and press enter.

Thanks for your time.

Lukas.