Given the n + 1 vertices x[0], y[0] .. x[N], y[N] of a simple polygon described in a clockwise direction, then the polygon's area can be calculated by:

abs( (sum(x[0]*y[1] + ... x[n-1]*y[n]) + x[N]*y[0]) -
     (sum(x[1]*y[0] + ... x[n]*y[n-1]) + x[0]*y[N])
   ) / 2
(Where abs returns the absolute value)

Task

Write a function/method/routine to use the the Shoelace formula to calculate the area of the polygon described by the ordered points:

     (3,4), (5,11), (12,8), (9,5), and (5,6) 



Show the answer here, on this page.




V := [[3, 4], [5, 11], [12, 8], [9, 5], [5, 6]]
 
n := V.Count()
for i, O in V
    Sum += V[i, 1] * V[i+1, 2]  -  V[i+1, 1] * V[i, 2]
MsgBox % result := Abs(Sum += V[n, 1] * V[1, 2]  -  V[1, 1] * V[n, 2]) / 2