import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.util.StringTokenizer;

public class Solution {

    /// GENERIC PART

    public static void main(String[] args) throws IOException {
        init(System.in);
        solve();
    }

    private static BufferedReader reader;
    private static StringTokenizer tokenizer;

    private static void init(InputStream input) {
        reader = new BufferedReader(new InputStreamReader(input));
        tokenizer = new StringTokenizer("");
    }

    private static String next() throws IOException {
        while (!tokenizer.hasMoreTokens()) {
            tokenizer = new StringTokenizer(reader.readLine());
        }
        return tokenizer.nextToken();
    }

    private static int nextInt() throws IOException {
        return Integer.parseInt(next(), 10);
    }

    private static long nextLong() throws IOException {
        return Long.parseLong(next(), 10);
    }

    private static int nextInt(int radix) throws IOException {
        return Integer.parseInt(next(), radix);
    }

    private static int[] nextInts(int n) throws IOException {
        final int[] a = new int[n];
        for (int i = 0; i < n; i++) {
            a[i] = nextInt();
        }
        return a;
    }

    private static double nextDouble() throws IOException {
        return Double.parseDouble(next());
    }

    /// SOLUTION PART

    private static void solve() throws IOException {
        int n = nextInt(), k = nextInt();
        int[] arr = nextInts(n);
        long c = 0;
        for (int i = k; i < arr.length; i += 2 * k + 1) {
            c += arr[i];
        }
        System.out.print(c);

    }


}