write a go solution for Description:
You are given a regular N-sided polygon. Label one arbitrary side as side 1, then label the next sides in clockwise order as side 2, 3, ..., N. There are A_i special points on side i. These points are positioned such that side i is divided into A_i+1 segments with equal length.

For instance, suppose that you have a regular 4-sided polygon, i.e., a square. The following illustration shows how the special points are located within each side when A=[3,1,4,6]. The uppermost side is labelled as side 1.

You want to create as many non-degenerate triangles as possible while satisfying the following requirements. Each triangle consists of 3 distinct special points (not necessarily from different sides) as its corners. Each special point can only become the corner of at most 1 triangle. All triangles must not intersect with each other.

Determine the maximum number of non-degenerate triangles that you can create.

A triangle is non-degenerate if it has a positive area.

Input Format:
The first line consists of an integer N (3<=N<=200,000).

The following line consists of N integers A_i (1<=A_i<=2*10^9).

Output Format:
Output a single integer representing the maximum number of non-degenerate triangles that you can create.

Note:
Explanation for the sample input/output #1

One possible construction which achieves maximum number of non-degenerate triangles can be seen in the following illustration.

Explanation for the sample input/output #2

One possible construction which achieves maximum number of non-degenerate triangles can be seen in the following illustration.. Output only the code with no comments, explanation, or additional text.