import sys
def input(): return sys.stdin.readline().rstrip()
def ii(f=int): return f(input())
def mi(f=int): return map(f, input().split())
def li(f=int): return list(map(f, mi()))

N, M = mi()
A = li()
B = li()
C = [max(0, A[i] - B[i]) for i in range(N)]
S = [0]
for c in C:
    S.append(S[-1] + c)


ans = -10**19
for i in range(N):
    if i + M <= N:
        ans = max(ans, S[i + M] - S[i])
print(ans)