Improve product income (stock)

I have always questioned the design format of the product inventory entry system.
For example, product A, currently has 15 pieces, but today we buy 20 new pieces of that product, the ideal would be to be able to add this last purchase value (20) and for the system to add the previous value (15) giving a total of 35 pieces of inventory.

However, the account must be done manually, I had 15, we bought 20 and in the deposit I must specify that I am 35, in small quantities there are no problems, but it is complicated to use when the amounts are of higher figures,

Example product B has 2356 inventory and 3678 were purchased since it requires the use of a calculator.
You have thought or is there another way that you can enter the new amount and add it to the value you had before?

Thank you