loop.with_index {|_, i|
    n = gets.to_i
    break if n == 0
    points = (1..n).map{ gets.split.map(&:to_i) }

    area = 0
    (points + [points[0]]).each_cons(2){|u, v|
        x1, y1 = u
        x2, y2 = v
        area += x1 * y2 - x2 * y1
    }
    printf("%d %.1f\n", i + 1, area.abs / 2.0)

    gets
}