def calc_l(px0,py0,px1,py1)
  ((px0-px1)**2+(py0-py1)**2)**0.5
end

def calc_s(px0,py0,px1,py1,px2,py2)
  a=calc_l(px0,py0,px1,py1)
  b=calc_l(px1,py1,px2,py2)
  c=calc_l(px2,py2,px0,py0)
  z=(a+b+c)/2
  (z*(z-a)*(z-b)*(z-c))**0.5
end

ps,s=[],0
while gets
  ps<<$_.split(",").map(&:to_f)
end
(ps.size-2).times do |i|
  s+=calc_s(ps[0][0],ps[0][1],ps[i+1][0],ps[i+1][1],ps[i+2][0],ps[i+2][1])
end
p s.round(6)