while (n=gets.to_i)!=0
  vs=n.times.map{gets.split.map(&:to_i)}
  vs.push(vs[0])
  puts ((vs.each_cons(2).map{|v| (v[0][0]-v[1][0])*(v[0][1]+v[1][1])}.inject(:+).abs / 2.0)*10).floor / 10.0
  gets
end