Sharepoint add value to another column without deleting the value

I am using a Sharepoint list; I have created 2 columns

1. receive item
2. in stock

If any value is added in (receive item), the code should show and add in (in stock) column without delete the item in (in stock). So if I edit same receive item again and add any other value it should add in (in stock) without deleting the previous value. For example, if I add 2 in (receive item) it should show in (in stock) and if I again edit the same (receive item) and add 10 so it should show 12 in (in stock).