o, *dots = readlines.map{|l| l.split(",").map(&:to_f) }
puts(dots.each_cons(2).map do |i,j|
  a = Math.sqrt((o[0]-i[0])**2 + (o[1]-i[1])**2)
  b = Math.sqrt((o[0]-j[0])**2 + (o[1]-j[1])**2)
  c = Math.sqrt((j[0]-i[0])**2 + (j[1]-i[1])**2)
  z = (a+b+c)/2
  Math.sqrt(z*(z-a)*(z-b)*(z-c))
end.inject(:+))