import sys
input = sys.stdin.readline

N,M=list(map(int,input().split()))
A=list(map(int,input().split()))
B=list(map(int,input().split()))


X=[]

for i in range(N):
    X.append(max(0,A[i]-B[i]))

S=[0]
for x in X:
    S.append(S[-1]+x)

ANS=0

for i in range(M,len(S)):
    ANS=max(ANS,S[i]-S[i-M])

print(ANS)