#include<stdio.h>
long long int a[200005], b[200005];
long long int sum[200005];
int main()
{
	long long int n, m;
	scanf("%lld %lld", &n, &m);
	long long int i;
	for (i = 0; i < n; i++)
		scanf("%lld", &a[i]);
	for (i = 0; i < n; i++)
		scanf("%lld", &b[i]);
	sum[0] = 0;
	for (i = 0; i < n; i++)
	{
		sum[i + 1] = sum[i];
		if (a[i] - b[i] > 0)
			sum[i + 1] += a[i] - b[i];
	}
	long long int ans = 0;
	for (i = 0; i <= n - m; i++)
		if (ans < sum[i + m] - sum[i])
			ans = sum[i + m] - sum[i];
	printf("%lld\n", ans);
	return 0;
}