Problem Statement  
You are given a circle of diameter d, with n points equally spaced around the circumference. The points are numbered in order around the circle 0, 1, 2, ... , n1. Of those n points, c of them are colored red. The points that are colored red are given by the generator function (g * k) % n, for k = 0, 1, 2, ..., c1. You are given ints d, n, c, and g. You are to return a double indicating the largest area of a quadrilateral formed from four of the colored points.  
Definition  
 
Notes  
  The % operator is the modulus (remainder) operator.  
  Notice that it makes no difference which point is point 0, and in which direction around the circle the points are numbered.  
Constraints  
  d will be between 1 and 1000, inclusive.  
  n will be between 4 and 1000000000, inclusive.  
  c will be between 4 and 500, inclusive.  
  c will be less than or equal to n.  
  g will be between 1 and n1, inclusive, and will be relatively prime to n.  
Examples  
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