from math import sin, cos, pi

n, l = map(int, input().split())
t = [2*pi * int(i) / l for i in input().split()]
t_sin = [sin(i) for i in t]
t_cos = [cos(i) for i in t]

a_sin = 0
a_cos = 0
for i in range(n):
    a_sin += t_sin[i] * (i-1)
    a_cos += t_cos[i] * (i-1)

b_sin = sum(t_sin)
b_cos = sum(t_cos)

ans = 0
for i in range(n):
    ans += t_sin[i] * a_cos - t_cos[i] * a_sin
    a_sin += n * t_sin[i] - b_sin
    a_cos += n * t_cos[i] - b_cos

print(ans * 3 / (n*(n-1)*(n-2)))