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

using namespace std;
using ll = long long;

int main() {
  ios::sync_with_stdio(false);
  cin.tie(nullptr);

  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];

  vector<ll> D(N);
  for (int i = 0; i < N; i++) D[i] = max(0, A[i] - B[i]);
  for (int i = 1; i < N; i++) D[i] += D[i - 1];

  ll ans = D[M - 1];
  for (int i = M; i < N; i++) {
    ans = max(ans, D[i] - D[i - M]);
  }
  cout << ans << '\n';
}