import sys
input = lambda : sys.stdin.readline().rstrip()

sys.setrecursionlimit(2*10**5+10)
write = lambda x: sys.stdout.write(x+"\n")
debug = lambda x: sys.stderr.write(x+"\n")
writef = lambda x: print("{:.12f}".format(x))


n,m = list(map(int, input().split()))
c = list(map(int, input().split()))
c.insert(0,0)
c.append(0)
a = [[0]*n for _ in range(n)]
b = c[:]
for i in range(n):
    for j in range(1,m+1):
        if i+j<n:
            a[i][i+j] += 1/m
        else:
            a[i][n-1-(j-(n-1-i))] += 1/m
    a[i][i] -= 1
a = np.array(a)[:-1, :-1]
import numpy as np
x = np.linalg.solve(a, -np.array(b[:-1])).tolist()
x.append(0)
inf = float("inf")
dp = [inf]*n
for i in range(n-m-1, n):
    dp[i] = c[i]
# print(dp)
for i in range(n-m-1)[::-1]:
    for j in range(1,m+1):
        dp[i] = min(dp[i], x[i+j]+c[i])
#     print(sum(dp[(i+1):(i+m+1)]), i+1, i+m)
    dp[i] = min(dp[i], sum(dp[i+1:i+m+1])/m + c[i])
ans = dp[0]
print(ans)