x, y = [], []

while line = gets
  _x, _y = line.split(',').map(&:to_f)
  x << _x
  y << _y
end

sum = 0
n = y.length

n.times do |i|
  if i.zero?
    _x = x[-1] - x[i + 1]
  elsif i == (n - 1)
    _x = x[i - 1] - x[0]
  else
    _x = x[i - 1] - x[i + 1]
  end

  sum += _x * y[i]
end

printf "%7.6f\n", sum / -2