index = 0
while (n=gets.to_i) != 0
  index += 1
  x = []
  y = []
  n.times do
    a,b = gets.chomp.split.map(&:to_i)
    x.push(a)
    y.push(b)
  end
  s = 0
  for i in 0..n-1
    if i < n-1
      s += (x[i]*y[i+1] - y[i]*x[i+1])
    elsif i = n-1
      s += (x[i]*y[0] - y[i]*x[0])
    end
  end
  s = s.abs / 2.0

  puts "#{index} #{s}"

  gets
end