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