def main():
    N, M = map(int, input().split())
    A = list(map(lambda a: max(int(a), 0), input().split()))
    
    sm = sum(A[:M])
    ans = sm
    for i in range(M, N):
        sm = sm + A[i] - A[i - M]
        ans = max(ans, sm)
    print(ans)

if __name__ == "__main__":
    main()