/* FILE: intervalSum.cpp   last change: 23-July-2012   author: Romeo Rizzi
 * a solver for problem 1 (IntervalSum) in the 23-07-2012 exam in Algorithms
 */

// #include <iostream> // NOTA: ho commentato questa riga per essere sicuro di non aver lasciato operazioni di input/output di debug nella fretta di consegnare, se compila cosi' non ci sono ed io non perdo stupidamente i miei punti-esame.
#include <cstdlib>
#include <fstream>
#include <cassert>

using namespace std;

const int MAX_N = 1000000;
int n, k, vect[MAX_N];

int maxSumFrom[MAX_N];   /* maxSumFrom[i] will store the maximum value of an interval starting in position i and of length at least k. */

int main() {
  ifstream fin("input.txt"); assert(fin);
  fin >> n >> k;
  for(int i = 0; i < n; i++)
    fin >> vect[i];
  fin.close();

  int best_so_far;
  int sum_of_k_from_pos, sum_all_from_pos = 0; // think of pos = n;
  if (k == 0) best_so_far = sum_of_k_from_pos = 0;
  for(int pos = n-1, length = 1; pos >= 0; pos--, length++ ) {  // length = the length of the interval from <pos> till the end of vect
     sum_all_from_pos += vect[pos];
     if(length == k)
       best_so_far = maxSumFrom[pos] = sum_of_k_from_pos = sum_all_from_pos;
     if(length > k) {
       sum_of_k_from_pos += vect[pos] - vect[pos+k]; 
       best_so_far = max( best_so_far,
                          maxSumFrom[pos] = max( sum_of_k_from_pos,
			                    vect[pos] + maxSumFrom[pos+1] )
			);
     }  
  }

  ofstream fout("output.txt"); assert(fout);
  fout << best_so_far;
  fout.close();
  return 0;
}