import java.util.*;

public class Main{
    public static void main(String[] args){
        Scanner scanner = new Scanner(System.in);

        int n = scanner.nextInt();
        int m = scanner.nextInt();

        int[] sums = new int[n+1];
        sums[0] = 0;
        for(int i = 1; i <= n; i++) {
            sums[i] = sums[i-1] + scanner.nextInt();
        }

        Deque<Integer> deque = new ArrayDeque<>();
        deque.offerLast(0);
        int ans = Integer.MIN_VALUE;
        for(int i = 1; i <= n; i++) {
            if(deque.isEmpty()) deque.offerLast(i);
            else {
                if(i - deque.peekFirst() > m) {
                    deque.removeFirst();
                }
                ans = Math.max(ans, sums[i] - sums[deque.peekFirst()]);
                while(!deque.isEmpty() &&sums[deque.peekLast()] >= sums[i]) {
                    deque.pollLast();
                }
                deque.offerLast(i);
            }
        }

        System.out.println(ans);
    }
}