Incorrect syntax near the keyword As

I opened a 'New Query' window in SSMS. I pasted the following code (well this is a simplified version but throws the same error as the real version).

Create function dbo.GetFull (@dbName varchar(100))
Returns double
As  --   <------------- underlined in red. 
Begin
	Declare @PercentFull double
	select @PercentFull = 0
	Return @PercentFull
End 
go