Number Data Entry (Same Cell)

Hi Guys!

I am looking to create an excel sheet where I enter in a cell (Cell A1) a number, which logs that amount in another cell (B2). Then when I enter in the cell (A1) another number, it adds to the amount calculated before.
So for instance, if I add 5 to A1 initially, then input 10 to A1 after, in B1 the amount should equal 15.

The reason I want to do this is to use this spreadsheet as a sheet to log the number of people walking through the doors of a venue who use specific peoples names/lists.
How do I do this?

Thanks in advance.