n, m = list(map(int, input().split()))
A = list(map(int, input().split()))
B = list(map(int, input().split()))
C = [0] * (n+1)
for i in range(n):
    C[i+1] = max(0, A[i]-B[i])+C[i]
ans = 0
for i in range(n-m+1):
    ans = max(ans, C[i+m]-C[i])
print(ans)