xy = []
readlines.map(&:chomp).each_with_index{|line, i|
  xy[i] = line.split(",").map(&:to_f)
}
sum = 0.0
a = Math.sqrt((xy[1][0]-xy[0][0])**2 + (xy[1][1]-xy[0][1])**2)
2.upto(xy.size-1){|i|
  b = Math.sqrt((xy[i][0]-xy[0][0])**2 + (xy[i][1]-xy[0][1])**2)
  c = Math.sqrt((xy[i][0]-xy[i-1][0])**2 + (xy[i][1]-xy[i-1][1])**2)
  z = (a + b + c) / 2.0
  sum += Math.sqrt(z*(z-a)*(z-b)*(z-c))
  a = b
}

puts "%.6f" % sum