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

from math import cos,sin,exp,pi

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

acc2 = [0j]+[i*sisuu[i] for i in range(n)]
for i in range(1,n+1):
    acc2[i] += acc2[i-1]

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

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