from math import cos,sin,pi
n,L = map(int,input().split())
*a, = map(int,input().split())
w = -2*pi/L

ans = x = y = v = 0j
for j in range(n):
    x += v
    y += (j-1)*v*2
    v = cos(w*a[j]) + 1j*sin(w*a[j])
    ans += (x*(n-2*j)+y)/v

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