#include <bits/stdc++.h>
using namespace std;
long long n, k;
int main() {
  ios::sync_with_stdio(false);
  cin.tie(0);

  cin >> n >> k;
  vector<long long> a(n);
  for (long long i = 0; i < n; ++i) cin >> a[i];
  long long s = 0;
  for (long long i = 0; i < k; ++i) s += a[i];
  long long res = s;
  for (long long i = k; i < n; ++i) res = max(res , s += a[i] - a[i - k]);
  cout << res;
  return 0;
}