Telescope

Time Limit: 5 Seconds
Memory Limit: 32768 KB

Dr. Extreme experimentally made an extremely precise telescope to investigate
extremely curious phenomena at an extremely distant place. In order to make
the telescope so precise as to investigate phenomena at such an extremely distant
place, even quite a small distortion is not allowed. However, he forgot the
influence of the internal gas affected by low-frequency vibration of magnetic
flux passing through the telescope. The cylinder of the telescope is not affected
by the low-frequency vibration, but the internal gas is.

The cross section of the telescope forms a perfect circle. If he forms a coil
by putting extremely thin wire along the (inner) circumference, he can measure
(the average vertical component of) the temporal variation of magnetic flux:
such measurement would be useful to estimate the influence. But points on the
circumference at which the wire can be fixed are limited; furthermore, the number
of special clips to fix the wire is also limited. To obtain the highest sensitivity,
he wishes to form a coil of a polygon shape with the largest area by stringing
the wire among carefully selected points on the circumference.

Your job is to write a program which reports the maximum area of all possible
m-polygons (polygons with exactly m vertices) each of whose vertices is one
of the n points given on a circumference with a radius of 1. An example of the
case n = 4 and m = 3 is illustrated below.

In the figure above, the equations such as "p1 = 0.0" indicate the
locations of the n given points, and the decimals such as "1.000000"
on m-polygons indicate the areas of m-polygons.

Parameter pi denotes the location of the i-th given point on the circumference
(1 <= i <= n). The location p of a point on the circumference is in the
range 0 <= p < 1, corresponding to the range of rotation angles from 0
to 2*PI radians. That is, the rotation angle of a point at p to the point at
0 equals 2*PI*p radians. (PI is the circular constant 3.14159265358979323846....)

You may rely on the fact that the area of an isosceles triangle ABC (AB = AC
= 1) with an interior angle BAC of �� radians (0 < a< PI) is sin(a)/2,
and the area of a polygon inside a circle with a radius of 1 is less than PI.

**Input**

The input consists of multiple subproblems followed by a line containing two
zeros that indicates the end of the input. Each subproblem is given in the following
format.

n m

p1 p2 ... pn

n is the number of points on the circumference (3 <= n <= 40). m is the
number of vertices to form m-polygons (3 <= m <= n). The locations of
n points, p1, p2, ..., pn, are given as decimals and they are separated by
either a space character or a newline. In addition, you may assume that 0 <=
p1 < p2 < ... < pn < 1.

**Output**

For each subproblem, the maximum area should be output, each in a separate line.
Each value in the output may not have an error greater than 0.000001 and its
fractional part should be represented by 6 decimal digits after the decimal
point.

**Sample Input**

4 3

0.0 0.25 0.5 0.666666666666666666667

4 4

0.0 0.25 0.5 0.75

30 15

0.00 0.03 0.06 0.09 0.12 0.15 0.18 0.21 0.24 0.27

0.30 0.33 0.36 0.39 0.42 0.45 0.48 0.51 0.54 0.57

0.61 0.64 0.66 0.69 0.72 0.75 0.78 0.81 0.84 0.87

40 20

0.351 0.353 0.355 0.357 0.359 0.361 0.363 0.365 0.367 0.369

0.371 0.373 0.375 0.377 0.379 0.381 0.383 0.385 0.387 0.389

0.611 0.613 0.615 0.617 0.619 0.621 0.623 0.625 0.627 0.629

0.631 0.633 0.635 0.637 0.639 0.641 0.643 0.645 0.647 0.649

0 0

**Sample Output**

1.183013

2.000000

3.026998

0.253581

Source:

**Asia 2000, Tsukuba (Japan)**
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