import java.util.*;


public class Main{

    void run(){
        int n = jin.nextInt();
        int m = jin.nextInt();
        nums.add(0);
        for (int i = 0 ; i < n ; i++) nums.add(jin.nextInt());
        for (int i = 1 ; i <= n ; i++) nums.set(i, nums.get(i)+nums.get(i-1));

        int res = Integer.MIN_VALUE;
        for (int i = 1; i <= n ; i++){
            while(!queue.isEmpty() && i - queue.peekFirst() > m) queue.removeFirst();

            if (!queue.isEmpty()) res = Math.max(res, nums.get(i) - nums.get(queue.peekFirst())); // why not peekF -1 ?
            else res = Math.max(res, nums.get(i));                                              // 差点漏掉了
            while(!queue.isEmpty() && nums.get(i) <= nums.get(queue.peekLast())) queue.removeLast();
            queue.offerLast(i);
        }
        res = Math.max(res, nums.get(n) - nums.get(queue.peekFirst()-1));
        System.out.println(res);
    }

    List<Integer> nums = new ArrayList<>();
    Deque<Integer> queue = new LinkedList<>();
    private Scanner jin = new Scanner(System.in);
    public static void main(String[] args) throws Exception {new Main().run();}
}