loop.with_index {|_, k|
    n = gets.to_i
    break if n == 0
    points = (1..n).map{ gets.split.map &:to_f }
    points << points[0]
    area = points.each_cons(2).map{|u, v|
        u[1] * v[0] - u[0] * v[1]
    }.inject(:+) / 2
    puts "#{k+1} %.1f" % area
    gets
}