import sys

# sys.setrecursionlimit(200005)
int1 = lambda x: int(x)-1
pDB = lambda *x: print(*x, end="\n", file=sys.stderr)
p2D = lambda x: print(*x, sep="\n", end="\n\n", file=sys.stderr)
def II(): return int(sys.stdin.readline())
def LI(): return list(map(int, sys.stdin.readline().split()))
def LLI(rows_number): return [LI() for _ in range(rows_number)]
def LI1(): return list(map(int1, sys.stdin.readline().split()))
def LLI1(rows_number): return [LI1() for _ in range(rows_number)]
def SI(): return sys.stdin.readline().rstrip()

dij = [(0, 1), (-1, 0), (0, -1), (1, 0)]
# dij = [(0, 1), (-1, 0), (0, -1), (1, 0), (1, 1), (1, -1), (-1, 1), (-1, -1)]
inf = (1 << 63)-1
# inf = (1 << 31)-1
# md = 10**9+7
md = 998244353

from math import *

n, l = LI()
tt = LI()

ss, cc = [], []
for t in tt:
    ss.append(sin(2*pi*t/l))
    cc.append(cos(2*pi*t/l))
ss *= 2
cc *= 2
# pDB(ss, cc)

rs = [0]
for s in ss: rs.append(rs[-1]+s)
rc = [0]
for c in cc: rc.append(rc[-1]+c)

k = -n+2
s = c = 0
for i in range(n-1):
    s += ss[i]*k
    c += cc[i]*k
    k += 2

ans = 0
for i in range(n, n+n):
    s += (ss[i-n]+ss[i-1])*(n-2)-2*(rs[i-1]-rs[i-n+1])
    c += (cc[i-n]+cc[i-1])*(n-2)-2*(rc[i-1]-rc[i-n+1])
    ans += ss[i]*c-cc[i]*s

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