gistfile1.txt

import math


class CollidingCircles(object):
    def get_expected_value(self, radii, steps_count):
        return math.pi * self.__get_expected_value_for_step(radii, steps_count)

    def __get_expected_value_for_step(self, radii_set, step_idx):
        if step_idx == 0:
            return sum(radius * radius for radius in radii_set)

        total_val = 0
        count = 0

        for i in range(len(radii_set)):
            for j in range(i + 1, len(radii_set)):
                new_list = radii_set[:i] + [radii_set[i] + radii_set[j]] + radii_set[i + 1: j] + radii_set[j+1:]
                total_val += self.__get_expected_value_for_step(new_list, step_idx - 1)
                count += 1

        return total_val * 1.0 / count


if __name__ == '__main__':
    n, k = map(int, input().strip().split(' '))
    r = list(map(int, input().strip().split(' ')))

    print(CollidingCircles().get_expected_value(r, k))