from itertools import accumulate

N, K, *A = map(int, open(0).read().split())

M = [a if a < 0 else 0 for a in A]
P = [a if a > 0 else 0 for a in A]

MA = [0] + list(accumulate(M))
PA = [0] + list(accumulate(P))

minus = max(q - p for p, q in zip(MA, MA[K:]))
plus = min(q - p for p, q in zip(PA, PA[K:]))

print(sum(P) + max(minus, -plus))