Angle between 3 given vertices in a n-sided regular polygon

Given a n-sided regular polygon and three vertices a1, a2 and a3, the task is to find the angle suspended at vertex a1 by vertex a2 and vertex a3.

Examples: 

Input: n = 6, a1 = 1, a2 = 2, a3 = 4
Output: 90

Input: n = 5, a1 = 1, a2 = 2, a3 = 5
Output: 36

Approach:  

1. The angle subtended by an edge on the center of n sided regular polygon is 360/n.
2. The angle subtended by vertices separated by k edges becomes (360*k)/n.
3. The chord between the vertices subtends an angle with half the value of the angle subtended at the center at the third vertex which is a point on the circumference on the circumcircle.
4. Let the angle obtained in this manner be a = (180*x)/n where k is number of edges between i and k.
5. Similarly for the opposite vertex we get the angle to be b = (180*y)/n where l is number of edges between j and k.
6. The angle between the three vertices thus equals 180-a-b.

Below is the implementation of the above approach:  

36

Time Complexity: O(1)

Auxiliary Space: O(1)