def func(a, b)
  a[0] * b[1] - a[1] * b[0]
end

cas = 1
while (n = gets.to_i) != 0
  arr = Array.new
  n.times do
    x, y = gets.split
    x = x.to_f
    y = y.to_f
    arr.push([x, y])
  end
  area = 0
  for i in (0...n) do
    area += func(arr[i-1], arr[i])
  end
  area = area.abs / 2
  puts "#{cas} #{area}"
  cas += 1
  gets
end

