require 'matrix'

data = $<.readlines.map {|l| Vector[*l.split(",").map(&:to_f)]}
square = 0
ym = data.map {|d| d[1]}.max
p0 = data.select {|d| d[1] == ym}.sort_by {|d| d[0]}.first
(data - [p0]).sort_by {|d| d[0]}.each_cons(2) do |p1, p2|
  a = (p1 - p0).norm
  b = (p2 - p0).norm
  c = (p2 - p1).norm
  z = (a + b + c) / 2
  square += Math.sqrt(z * (z - a) * (z - b) * (z - c))
end
puts square
