Which of the following is not considered a part of Circle of Willis?
Circle of Willi's is formed by:-

In front: anterior communicating artery.
Anterolaterally: two anterior cerebral artery.
Laterally: proximal segments of both internal carotid arteries.
Postero-laterally: two posterior communicating arteries.
Behind: proximal segments of both posterior cerebral arteries, arising at bifurcating terminal of the basilar artery.
Anterior cerebral artery
Middle cerebral artery
Posterior communicating artery
Posterior cerebral artery