## https://yukicoder.me/problems/no/1385

import math

def main():
    N, L = map(int, input().split())
    T = list(map(int, input().split()))

    # sin, cosの累積和
    f_list = [0] * N
    g_list = [0] * N
    f = 0
    g = 0
    for i in reversed(range(N)):
        t = (2 * math.pi * T[i]) / L
        f += math.sin(t)
        g += math.cos(t)
        f_list[i] = f
        g_list[i] = g

    # 複雑な計算式の累積和
    x_cum_list = [0] * N
    x_cum = 0
    xcos_cum_list = [0] * N
    xcos_cum = 0
    xsin_cum_list = [0] * N
    xsin_cum = 0
    ysin_cum_list = [0] * N
    ysin_cum = 0
    ycos_cum_list = [0] * N
    ycos_cum = 0
    for i in reversed(range(N - 1)):
        f = f_list[i + 1]
        g = g_list[i + 1]
        t = (2 * math.pi * T[i]) / L
        cos_t = math.cos(t)
        sin_t = math.sin(t)

        x = f * cos_t - g * sin_t
        y = (N - 1 - i) - g * cos_t - f * sin_t

        x_cum += x
        x_cum_list[i] = x_cum
        xcos_cum += x * cos_t
        xcos_cum_list[i] = xcos_cum
        xsin_cum += x * sin_t
        xsin_cum_list[i] = xsin_cum
        ysin_cum += y * sin_t
        ysin_cum_list[i] = ysin_cum
        ycos_cum += y * cos_t
        ycos_cum_list[i] = ycos_cum

    # 
    cum_value_list = [0] * N
    for i in reversed(range(N - 2)):
        t = (2 * math.pi * T[i]) / L
        cos_t = math.cos(t)
        sin_t = math.sin(t)

        value = x_cum_list[i + 1] / 2
        value -= xcos_cum_list[i + 1] * cos_t / 2
        value -= xsin_cum_list[i + 1] * sin_t / 2
        value += ysin_cum_list[i + 1] * cos_t / 2
        value -= ycos_cum_list[i + 1] * sin_t / 2
        cum_value_list[i] = value

    ans = (N * (N - 1) * (N - 2)) // 6
    answer = 0
    for i in range(N):
        answer += cum_value_list[i] / ans 
    print(answer)






if __name__ == "__main__":
    main()