point = Array.new

def solve(point)
  source = point[0]
  point.delete_at(0)
#  point.sort{|p1,p2| p1[1] <=> p2[1]}
    
  sum = 0
  while point.size > 1 do
    p1 = point[0]
    p2 = point[1]
    
    a = getDistance(source, p1)
    b = getDistance(source,p2)
    c = getDistance(p1, p2)
    z = (a + b + c) / 2
      
    sum += getArea(a, b, c, z)
    
    #不要になった座標は削除する
    point.delete_at(0)
  end
  sum
end

def getDistance(p1, p2)
  Math.sqrt((p1[0] - p2[0]) ** 2 + (p1[1] - p2[1]) ** 2)
end

def getArea(l1, l2, l3, z)
  Math.sqrt(z * (z - l1) * (z - l2) * (z - l3))
end

while input = $stdin.gets do
  point << input.chomp.split(",").map(&:to_i)
end

printf "%.6f\n", solve(point)