require 'matrix'

p0, *points = $<.readlines.map {|l| Vector[*l.split(",").map(&:to_f)]}
result = points.each_cons(2).map do |p1, p2|
  a = (p1 - p0).norm
  b = (p2 - p0).norm
  c = (p2 - p1).norm
  z = (a + b + c) / 2
  Math.sqrt(z * (z - a) * (z - b) * (z - c))
end.sum
puts result
