import java.io.BufferedReader;
import java.io.BufferedWriter;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.OutputStreamWriter;
import java.util.ArrayList;
import java.util.List;
import java.util.stream.Stream;

public class Main {
    static final BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
    static final BufferedWriter bw = new BufferedWriter(new OutputStreamWriter(System.out));

    public static void main(String[] args) {
        try {
            IRunner runner = new Runner(br, bw);
            runner.run();
            runner.flush();

            br.close();
            bw.close();
        } catch (IOException e) {
            throw new RuntimeException(e);
        }
    }
}

class Runner implements IRunner {
    final IReader reader;
    final BufferedWriter bw;
    final StringBuilder sb = new StringBuilder();

    final int N, K;
    final int[] numbers;
    final int[] cumuls; // sigma(0, i)

    Runner(BufferedReader br, BufferedWriter bw) {
        this.reader = new Reader(br);
        this.bw = bw;

        try {
            var _nk = reader.readInts();
            N = _nk[0];
            K = _nk[1];
            sb.ensureCapacity(10);

            numbers = reader.readInts(); // size N
            cumuls = new int[N];

            cumuls[0] = numbers[0];
            for (int i = 1; i < N; ++i) {
                cumuls[i] = numbers[i] + cumuls[i - 1];
            }
        } catch (IOException e) {
            throw new RuntimeException(e);
        }
    }

    @Override
    public void flush() {
        try {
            bw.write(sb.toString());
            bw.flush();
        } catch (IOException e) {
            throw new RuntimeException(e);
        }
    }

    @Override
    public void run() throws IOException {
        int max = sum(0, K - 1);
        for (int i = 1; i < N; ++i) {
            int j = i + K - 1;
            if (j >= N) {
                break;
            }
            max = Math.max(max, sum(i, j));
        }
        sb.append(max).append('\n');
    }

    private int sum(int iFrom, int iTo) {
        if (iFrom <= 0) {
            return cumuls[iTo];
        }
        return cumuls[iTo] - cumuls[iFrom - 1];
    }
}

class Reader implements IReader {
    private BufferedReader br;

    public Reader(BufferedReader br) {
        this.br = br;
    }

    @Override
    public void skipLine() throws IOException {
        br.readLine();
    }

    @Override
    public Stream<String> lines() throws IOException {
        return br.lines();
    }

    @Override
    public String line() throws IOException {
        return br.readLine();
    }

    @Override
    public int[] readInts() throws IOException {
        String[] tokens = br.readLine().split(" ");
        int[] ints = new int[tokens.length];
        for (int i = 0; i < tokens.length; ++i) {
            ints[i] = Integer.parseInt(tokens[i]);
        }
        return ints;
    }

    @Override
    public List<Integer> readIntegers() throws IOException {
        String[] tokens = br.readLine().split(" ");
        var res = new ArrayList<Integer>(tokens.length);
        for (String token : tokens) {
            res.add(Integer.valueOf(token));
        }
        return res;
    }
}

interface IRunner {
    void run() throws IOException;

    void flush();
}

interface IReader {
    void skipLine() throws IOException;

    Stream<String> lines() throws IOException;

    String line() throws IOException;

    List<Integer> readIntegers() throws IOException;

    int[] readInts() throws IOException;
}