Getting the line number of the first line in the editor

I am working  on a Visual Studio Editor Extension and have the following question.

I'm trying the figure out how I can get the line number of the first line that is displayed in the VS editor. So, when the user scrolls down the first line number is changed from 1 to 2 to 3 etc. Can this be accomplished by using some property of the IWpfTextView?