Decreasing a cell value by one

Hi everyone:

I'm a complete Excel idiot, I have no knowledge of macros or VBA, so please forgive me if there is an obvious answer to my question.

I use a simple excel spreadsheet to manage the inventory of a business. I use it to track the current stock. So whenever there is a sale, I will manually reduce the quantity of stock.

What I am trying to do in my spreadsheet (which I've attached a small screenshot if it helps anyone) is to be able to use a keystroke to increase or decrease the value of a cell by one. I.e. pressing "," will increase the number in a highlighted/selected cell by one and pressing "." will decrease the number in a highlighted/selected cell by one.

At the moment I am re-inputting the values after doing the math in my head. So when a customer buys 7 pants, I'll have to calculate in my head and then key in the new value in the cell. This is time consuming and also makes it open to mistakes. Is there a simple way to do this? Can anyone help me?

Thank you very much in advance!!