output_description: For each data set, your program should output its sequence number (1 for the
  first data set, 2 for the second, etc.) and the area of the polygon separated
  by a single space. The area should be printed with one digit to the right of
  the decimal point.
  
  The sequence number and the area should be printed on the same line. Since
  your result is checked by an automatic grading program, you should not insert
  any extra characters nor lines on the output.
input_description: The input contains multiple data sets, each representing a polygon. A data set
  is given in the following format.
  
      
      
         n
         x1 y1
         x2 y2
         ...
         xn yn
      
  
  The first integer n is the number of vertices, such that 3 <= n <= 50. The
  coordinate of a vertex pi is given by (xi, yi). xi and yi are integers between
  0 and 1000 inclusive. The coordinates of vertices are given in the order of
  clockwise visit of them.
  
  The end of input is indicated by a data set with 0 as the value of n.
attempt: while 1:
      n = int(input())
      if n == 0:
          break
      p = [complex(*map(int, input().split())) for _ in range(n)]
      s, pre = 0, p[0]
      while p:
          now = p.pop()
          s += now.real * pre.imag - now.imag * pre.real
          pre = now
      print(abs(s) / 2)
      input()
  
problem_description: Area of Polygons
  
  Polygons are the most fundamental objects in geometric processing. Complex
  figures are often represented and handled as polygons with many short sides.
  If you are interested in the processing of geometric data, you'd better try
  some programming exercises about basic operations on polygons.
  
  Your job in this problem is to write a program that computes the area of
  polygons.
  
  A polygon is represented by a sequence of points that are its vertices. If the
  vertices p1, p2, ..., pn are given, line segments connecting pi and pi+1 (1 <=
  i <= n-1) are sides of the polygon. The line segment connecting pn and p1 is
  also a side of the polygon.
  
  You can assume that the polygon is not degenerate. Namely, the following facts
  can be assumed without any input data checking.
  
    * No point will occur as a vertex more than once.
    * Two sides can intersect only at a common endpoint (vertex).
    * The polygon has at least 3 vertices.
  
  Note that the polygon is not necessarily convex. In other words, an inner
  angle may be larger than 180 degrees.
samples: [{"input": "1 1\n    3 4\n    6 0\n    \n    7\n    0 0\n    10 10\n    0 20\n    10 30\n    0 40\n    100 40\n    100 0\n    \n    0", "output": "8.5\n    2 3800.0"}]