#pragma GCC optimize ("O3")
#include <bits/stdc++.h>
using namespace std;
const long long nmax = 1e6 + 5;
long long n , k , res = 0;
int main(){
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);cout.tie(NULL);
  cin >> n >> k;
    vector<int> a(n);
    for (int &x : a) cin >> x;

    long long sum = 0;
    for (int i = 0; i < k; ++i) sum += a[i];

    long long res = sum;
    for (int i = k; i < n; ++i) res = max(res, sum += a[i] - a[i - k]);

    cout << res;
}