Xamarin Forms Android - Create Folder into External Storage

Hi,

I'm trying to create a public folder accessible to anyone that browse the device files.
I'm following the instructions of Microsoft, but i'm facing the first obstacle.
When i try to get the directory of External Storage with

var path= Android.OS.Environment.ExternalStorageDirectory.ToString();

i see the error:

Android does not contain a definition for 'OS'.

Can anyone help me?

Thanks.