# Enter your code here. Read input from STDIN. Print output to STDOUT
import sys
LINES = sys.stdin.readlines()
n,k = map(int, LINES[0].split())
c = map(int, LINES[1].split())

def gauss(A):
    n = len(A)

    for i in range(0, n):
        # Search for maximum in this column
        maxEl = abs(A[i][i])
        maxRow = i
        for k in range(i+1, n):
            if abs(A[k][i]) > maxEl:
                maxEl = abs(A[k][i])
                maxRow = k

        # Swap maximum row with current row (column by column)
        for k in range(i, n+1):
            tmp = A[maxRow][k]
            A[maxRow][k] = A[i][k]
            A[i][k] = tmp

        # Make all rows below this one 0 in current column
        for k in range(i+1, n):
            c = -A[k][i]/A[i][i]
            for j in range(i, n+1):
                if i == j:
                    A[k][j] = 0
                else:
                    A[k][j] += c * A[i][j]

    # Solve equation Ax=b for an upper triangular matrix A
    return A
#    x = [0 for i in range(n)]
#    for i in range(n-1, -1, -1):
#        x[i] = A[i][n]/A[i][i]
#        for k in range(i-1, -1, -1):
#            A[k][n] -= A[k][i] * x[i]
#    return x

A = []
for i in xrange(n):
    z = []
    for j in xrange(n):
        if abs(i-j) <= k:
            z.append(1)
        else:
            z.append(0)
    z.append(1)
    A.append(z)

#for i in xrange(n):
#    for j in xrange(n+1):
#        print A[i][j], " ",
#    print ""

A = gauss(A)

#print "Gauss"
#for i in xrange(n):
#    for j in xrange(n+1):
#        print A[i][j], " ",
#    print ""

n = len(A)
#A[n-1][n-1] = 1

x = [0 for i in range(n)]
for i in range(n-1, -1, -1):
    x[i] = A[i][n]/A[i][i]
    for k in range(i-1, -1, -1):
        A[k][n] -= A[k][i] * x[i]

sc = 0
for i in xrange(n):
    if x[i] > 0:
        sc += c[i]
print sc