Need a Formula for SUM for individual cell to add by entering number and then press enter

I have an excel workbook that I'm working on and I'm trying to see if there is a SUM function or COUNT function or Formula that I can use that will quickly add a number up within an individual cell. So to give you an example, cell B7 , B8 and B9. B7 currently has the number 4 in it. However, throughout the day I want to be able to add to that number. Instead of just doing the math in my head and entering the new sum over the number 4
that's currently in there, I would instead just like to enter a "2" in that cell where the number 4 currently is and then hit enter and then this formula that I am looking for will automatically add the number 4 that was already there with the new number 2 and now have the new sum of "6" in that cell after I hit enter. Then later on I may need to add another number 2 where the cell now has a 6 and hit enter and then get an "8" I'm almost positive there is a way to format this cell so it will just continuously add to the current number throughout the day. Please let me know if you have any suggestions? Thanks, Rich

I believe I am on Mac Excel 2008. I guess I thought there was a function I could format the cell with that would just allow me to keep adding to the current cell by just putting in the new number that I would like to add to the current number and it would automatically update the cell with the new SUM. Any help is appreciated. Thanks, Rich