#include <iostream>
#include <vector>
#include <algorithm>

using namespace std;

int main()
{
  int n, m;
  cin >> n >> m;
  vector<int> a(n), b(n);
  for (int i = 0; i < n; ++i) cin >> a[i];
  for (int i = 0; i < n; ++i) cin >> b[i];
  for (int i = 0; i < n; ++i) a[i] = max(a[i] - b[i], 0);
  vector<long long> sum(n + 1);
  for (int i = 0; i < n; ++i) sum[i + 1] = sum[i] + a[i];
  long long ans = 0;
  for (int i = m; i <= n; ++i) ans = max(ans, sum[i] - sum[i - m]);
  cout << ans << endl;
}