from math import sin, cos, pi

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

t.sort()

s_sin = 0
s_cos = 0
for i in range(n):
    s_sin += sin(t[i]) * (i-1)
    s_cos += cos(t[i]) * (i-1)

t_sin = 0
t_cos = 0
for i in range(n):
    t_sin += sin(t[i])
    t_cos += cos(t[i])

ans = 0
xx = 0
for i in range(n):
    ans += sin(t[i]) * s_cos - cos(t[i]) * s_sin
    s_sin += n * sin(t[i]) - t_sin
    s_cos += n * cos(t[i]) - t_cos

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