import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.util.StringTokenizer;
/**
* 수열
*/
public class Main {
static int N, K, result;
static int[] temp;
public static void main(String[] args) throws IOException {
BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
StringTokenizer st = new StringTokenizer(br.readLine());
N = Integer.parseInt(st.nextToken());
K = Integer.parseInt(st.nextToken());
temp = new int[N];
st = new StringTokenizer(br.readLine());
for (int i = 0; i < N; i++) {
temp[i] = Integer.parseInt(st.nextToken());
}
int sum = 0;
for (int i = 0; i < K; i++) {
sum += temp[i];
}
int r = K;
result = sum;
for (int i = 0; i < N - K; i++) {
sum -= temp[i];
sum += temp[r++];
result = Math.max(result, sum);
}
System.out.println(result);
}
}