Solve the following sql problem:
What is the average annual amount of shipped sales from 1997 to 1998?

Schema:
SCHEMA:
- Table: Categories
  - Column: CategoryID
    - Samples: [1, 2, 3, 4, 5]
  - Column: CategoryName
    - Samples: [Beverages, Condiments, Confections, Dairy Products, Grains/Cereals]
  - Column: Description
    - Samples: [Soft drinks, coffees, teas, beers, and ales, Sweet and savory sauces, relishes, spreads, and seasonings, Desserts, candies, and sweet breads, Cheeses, Breads, crackers, pasta, and cereal]
- Table: Customers
  - Column: CustomerID
    - Samples: [1, 2, 3, 4, 5]
  - Column: CustomerName
    - Samples: [Alfreds Futterkiste, Ana Trujillo Emparedados y helados, Antonio Moreno Taquería, Around the Horn, Berglunds snabbköp]
  - Column: ContactName
    - Samples: [Maria Anders, Ana Trujillo, Antonio Moreno, Thomas Hardy, Christina Berglund]
  - Column: Address
    - Samples: [Obere Str. 57, Avda. de la Constitución 2222, Mataderos 2312, 120 Hanover Sq., Berguvsvägen 8]
  - Column: City
    - Samples: [Berlin, México D.F., México D.F., London, Luleå]
  - Column: PostalCode
    - Samples: [12209, 5021, 5023, WA1 1DP, S-958 22]
  - Column: Country
    - Samples: [Germany, Mexico, Mexico, UK, Sweden]
- Table: Employees
  - Column: EmployeeID
    - Samples: [1, 2, 3, 4, 5]
  - Column: LastName
    - Samples: [Davolio, Fuller, Leverling, Peacock, Buchanan]
  - Column: FirstName
    - Samples: [Nancy, Andrew, Janet, Margaret, Steven]
  - Column: BirthDate
    - Samples: [1968-12-08, 1952-02-19, 1963-08-30, 1958-09-19, 1955-03-04]
  - Column: Photo
    - Samples: [EmpID1.pic, EmpID2.pic, EmpID3.pic, EmpID4.pic, EmpID5.pic]
  - Column: Notes
    - Samples: [Education includes a BA in psychology from Colorado State University. ..., Andrew received his BTS commercial and a Ph.D. in international market..., Janet has a BS degree in chemistry from Boston College). She has also ..., Margaret holds a BA in English literature from Concordia College and a..., Steven Buchanan graduated from St. Andrews University, Scotland, with ... (truncated)]
- Table: Shippers
  - Column: ShipperID
    - Samples: [1, 2, 3]
  - Column: ShipperName
    - Samples: [Speedy Express, United Package, Federal Shipping]
  - Column: Phone
    - Samples: [(503) 555-9831, (503) 555-3199, (503) 555-9931]
- Table: Suppliers
  - Column: SupplierID
    - Samples: [1, 2, 3, 4, 5]
  - Column: SupplierName
    - Samples: [Exotic Liquid, New Orleans Cajun Delights, Grandma Kelly's Homestead, Tokyo Traders, Cooperativa de Quesos 'Las Cabras']
  - Column: ContactName
    - Samples: [Charlotte Cooper, Shelley Burke, Regina Murphy, Yoshi Nagase, Antonio del Valle Saavedra]
  - Column: Address
    - Samples: [49 Gilbert St., P.O. Box 78934, 707 Oxford Rd., 9-8 Sekimai Musashino-shi, Calle del Rosal 4]
  - Column: City
    - Samples: [Londona, New Orleans, Ann Arbor, Tokyo, Oviedo]
  - Column: PostalCode
    - Samples: [EC1 4SD, 70117, 48104, 100, 33007]
  - Column: Country
    - Samples: [UK, USA, USA, Japan, Spain]
  - Column: Phone
    - Samples: [(171) 555-2222, (100) 555-4822, (313) 555-5735, (03) 3555-5011, (98) 598 76 54]
- Table: Products
  - Column: ProductID
    - Samples: [1, 2, 3, 4, 5]
  - Column: ProductName
    - Samples: [Chais, Chang, Aniseed Syrup, Chef Anton's Cajun Seasoning, Chef Anton's Gumbo Mix]
  - Column: SupplierID
    - Samples: [1, 1, 1, 2, 2]
  - Column: CategoryID
    - Samples: [1, 1, 2, 2, 2]
  - Column: Unit
    - Samples: [10 boxes x 20 bags, 24 - 12 oz bottles, 12 - 550 ml bottles, 48 - 6 oz jars, 36 boxes]
  - Column: Price
    - Samples: [18.0, 19.0, 10.0, 22.0, 21.35]
- Table: Orders
  - Column: OrderID
    - Samples: [10248, 10249, 10250, 10251, 10252]
  - Column: CustomerID
    - Samples: [90, 81, 34, 84, 76]
  - Column: EmployeeID
    - Samples: [5, 6, 4, 3, 4]
  - Column: OrderDate
    - Samples: [1996-07-04, 1996-07-05, 1996-07-08, 1996-07-08, 1996-07-09]
  - Column: ShipperID
    - Samples: [3, 1, 2, 1, 2]
- Table: OrderDetails
  - Column: OrderDetailID
    - Samples: [1, 2, 3, 4, 5]
  - Column: OrderID
    - Samples: [10248, 10248, 10248, 10249, 10249]
  - Column: ProductID
    - Samples: [11, 42, 72, 14, 51]
  - Column: Quantity
    - Samples: [12, 10, 5, 9, 40]

SELECT SUM(T2.UnitPrice * T2.Quantity * (1 - T2.Discount)) / 3 FROM Orders AS T1 INNER JOIN `Order Details` AS T2 ON T1.OrderID = T2.OrderID WHERE T1.ShippedDate BETWEEN '1996-01-01 00:00:00' AND '1998-12-31 23:59:59'