# Enter your code here. Read input from STDIN. Print output to STDOUT
n,k  = map(int,raw_input().split(' '))
c = map(int,raw_input().split(' '))

def recurse(c,k):
    n = len(c)
    if n ==1:
        return c[0]
    else:
        runsum = [0]*n
        for i in range(n):
            for j in range(-k,k+1):
                if (i+j>=0) and (i+j<n):
                    runsum[i] += c[i+j]
        ct = [0]*n
        for i in range(n):
            ct[i] = float(c[i])/runsum[i]
        (m,idx) = min((v,i) for i,v in enumerate(ct))
        c0 = c[idx]
        p1 = idx-k
        p2 = idx+k
        if p1>0:
            c0 += recurse(c[:p1],k)
        if p2<n-1:
            c0 += recurse(c[p2:],k)
        return c0

print recurse(c,k)