import numpy as np
from numpy.fft import rfft, irfft
from math import pi

n, l = map(int, input().split())
t = np.array(list(map(int, input().split()))) * (2*pi/l)
sin = np.sin(t)
cos = np.cos(t)

x = irfft(rfft(cos)*rfft(sin)) * np.arange(n)
print(np.sum(x)*0+0.5)