How to add the computed column to a table based on the columns already exist in the table?

The application contains a table that has the following definition:

CREATE TABLE Items
(ItemID int identity(1,1) NOT NULL PRIMARY KEY,
ItemsInStore int NOT NULL,
ItemsInWarehouse int NOT NULL)

You need to create a computed column that returns the sum total of the ItemsInStore and ItemsInWarehouse values for each row.

ItemID	ItemsInStore	ItemsInWarehouse
1	30	35
2	400	50
3	11	3

Alter table Items
Add TotalItems AS ItemsInStore + ItemsInWarehouse

ItemID	ItemsInStore	ItemsInWarehouse	TotalItems
1	30	35	65
2	400	50	450
3	11	3	14