### subroutines

def area(p0, p1, p2)
  v1x = p1[0] - p0[0]
  v1y = p1[1] - p0[1]
  v2x = p2[0] - p0[0]
  v2y = p2[1] - p0[1]
  (v1y * v2x - v1x * v2y) / 2
end

### main

id = 0

loop do
  line = ''
  line = gets.strip while line.empty?
  n = line.to_i
  break if n == 0

  pts = n.times.map{gets.split.map(&:to_f)}
  p0 = pts.shift
  #p [p0, pts]

  sum = 0.0

  for i in (0...(n - 2))
    pi = pts[i]
    pj = pts[i + 1]
    sum += area(p0, pi, pj)
  end

  id += 1
  puts [id, "%.1f" % sum].join(' ')
end