import sys
import numpy as np

read = sys.stdin.buffer.read
readline = sys.stdin.buffer.readline
readlines = sys.stdin.buffer.readlines

# 出発時に支払うことに統一

N, M = map(int, readline().split())
C = np.array([0] + read().split() + [0], np.float64)

p = 1 / M
mat = np.eye(N)
for n in range(N - 1):
    for i in range(1, M + 1):
        to = n + i
        if to > N - 1:
            to = 2 * (N - 1) - to
        mat[n, to] -= p
dp_0 = np.linalg.solve(mat, C)

dp_1 = np.zeros(N)
for n in range(N - 1, -1, -1):
    # 魔法権利があるときのコスト期待値
    if n + M >= N - 1:
        dp_1[n] = C[n]
    else:
        # 魔法を使う
        x = dp_0[n + 1:n + M + 1].min()
        # 適当にすすむ
        y = dp_1[n + 1:n + M + 1].mean()
        dp_1[n] = min(x, y) + C[n]

print(dp_1[0])