N,M = map(int,input().split())
A = list(map(int,input().split()))
B = list(map(int,input().split()))
ans = 0
l = []
for i in range(N):
    l.append(max(0,A[i]-B[i]))

tmp = sum(l[:M])
for i in range(N-M+1):
    if i != 0:
        tmp -= l[i-1]
        tmp += l[M+i-1]
    ans = max(tmp,ans)

print(ans)