n,L = map(int,input().split())
*a, = map(int,input().split())

from math import cos,sin,exp,pi
from itertools import accumulate

sisuu = [cos(-2*pi/L*ai) + 1j*sin(-2*pi/L*ai)  for ai in a]
acc = [0j]+sisuu[:]
for i in range(1,n+1):
    acc[i] += acc[i-1]

acc2 = [0j]+[i*(cos(-2*pi/L*a[i]) + i*1j*sin(-2*pi/L*a[i])) for i in range(n)]
for i in range(1,n+1):
    acc2[i] += acc2[i-1]

#print(sisuu)
#print(acc)
#print(acc2)

ans = 0j
for j in range(1,n):
    x = acc[i]*(n-2*j)
    y = acc2[i]*2
    ans += (x+y)/sisuu[j]

p = 3/n/(n-1)/(n-2)
print(abs(ans)*p)



"""
ans = 0
from math import sin,pi
for i in range(n):
    for j in range(i+1,n):
        ans += (n-2-2*(j-i-1))*sin((a[j]-a[i])*2*pi/L)

p = 3/n/(n-1)/(n-2)
print(ans)
"""