What kind of adapter do I need to connect a Vivitar lens to a D3100?

I have an old Vivitar telephoto lens that I would like to use on my Nikon D3100. Unfortunately, I'm not sure how to tell what type of lens adapter I need. The link above is a gallery of the lens and packaging.

The D3100 should be a Nikon F mount; they should compatible.

A careful examination of the rear of the lens shows that it is an M42 screw-mount so all you'll need is an M42 to Nikon adapter.http://www.rugift.com/photocameras/nikon_lens_adapter.htm

Note that you will have to manually focus the lens and likely have to use stop-down metering since there will be NO communication between lens and camera.