 = 1

while (n = gets.to_i) != 0 do
  vertices = []

  n.times do
    vertices.push(gets.chomp.split(/ /).map(&:to_i))
  end

  area = (0...vertices.size).map {|i|
    (x1, y1) = vertices[i]

    if i == vertices.size - 1
      (x2, y2) = vertices[0]
    else
      (x2, y2) = vertices[i + 1]
    end

    (x1 * y2 - x2 * y1)
  }.inject(:+).to_f / 2

  puts "#{x} #{area.abs}"

  x += 1

  gets
end