/* FILE: intervalSumSlow.cpp   last change: 23-July-2012   author: Romeo Rizzi
 * a solver for problem 1 (IntervalSum) in the 23-07-2012 exam in Algorithms
 * a slow (quadratic) solver which computes the value of each possible feasible solution, employed for tuning the scores
 * at least it is not cubic since it avoids recomputing each value separately from scratch
 * here, this spearing from the cubic trivial solution is achieved by employing a prefix sum vector
 * this appears to be a simplest/safest possible code, thus useful for assessing correctness by comparisons of the outputs. 
*/

// #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 sum_of_the_first[MAX_N +1];
// sum_of_the_first[i] stores the sum of the first i elements 

int main() {
  sum_of_the_first[0] = 0;
  ifstream fin("input.txt"); assert(fin);
  fin >> n >> k;
  for(int i = 0; i < n; i++) {
    fin >> vect[i];
    sum_of_the_first[i+1] = sum_of_the_first[i] + vect[i];
  }
  fin.close();

  int best_so_far =  sum_of_the_first[k];
  for(int first = 0; first+k-1 < n; first++ )
    for(int last = first+k-1; last < n; last++ )
      best_so_far = max( best_so_far, sum_of_the_first[last+1] - sum_of_the_first[first]);

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