#!/usr/bin/env python
# -*- coding:utf-8 -*-

def file_open(filename):
    T = 0
    caseList = []
    with open(filename) as f:
        T = int(f.readline().strip())
        for i in range(T):
            key = int(f.readline().strip())
            value = [int(x) for x in f.readline().strip().split()]
            caseList.append((key, value))
    return caseList

def sortforthis(elements):
    """
    >>> sortforthis([457, 478, 106, 601, 804])
    [804, 601, 457, 106, 478]
    >>> sortforthis([384, 675, 498, 839])
    [839, 675, 384, 498]
    """
    output = []
    elements = sorted(elements, reverse = True)
    is_even = False
    if len(elements) % 2 == 0:
        is_even = True
    else:
        is_even = False
    if is_even:
        output.append(elements[0])
        for i in range(1, len(elements), 2):
            output.append(elements[i])
        for i in range(len(elements) - 2 , 1, -2):
            output.append(elements[i])
    else:
        output.append(elements[0])
        for i in range(1, len(elements) -1, 2):
            output.append(elements[i])
        for i in range(len(elements) - 1, 1, -2):
            output.append(elements[i])
    return output

def process(caseTuple):
    import math
    menseki = 0
    numOfElements, elementsList = caseTuple
    angle = 2 * math.pi / numOfElements
    elementsList = sortforthis(elementsList)
    for i in range(numOfElements):
        j = i + 1 if i < numOfElements - 1 else 0
        tmp = math.sin(angle) * elementsList[i] * elementsList[j] / 2
        menseki += tmp
    return menseki

def solve(caseList, outputName):
    f = open(outputName, 'w')
    for i, T in enumerate(caseList):
        answer = process(T)
        f.write("Case #" + str(i + 1) + ": " + str(answer) + "\n")
    f.close()

def _test():
    import doctest
    doctest.testmod()

def main():
    FILENAME = "A-large.in"
    OUTPUTNAME = "outputAlarge.txt"
    A = file_open(FILENAME)
    solve(A, OUTPUTNAME)

if __name__ == "__main__":
    main()
    #_test()