k = 1
loop do
  n = gets.to_i
  break if n == 0
  p = []
  n.times{ p << gets.split.map(&:to_f) }
  n.times{ |i| p[i] += p[(i+1) % n] }
  s = p.map{ |x0, x1, y0, y1| x1*y0 - x0*y1 }.inject(:+) / 2
  puts "#{k} %.1f" % s
  k += 1
  gets
end