import java.io.*;
import java.util.*;

class Main {

    private static PrintWriter out;

    private static <T> void mprintln(T... ar) {
        for (T i : ar) {
            out.print(i + " ");
        }
        out.println();
    }

    public static void main(String[] args) throws FileNotFoundException {

        // Input from file
        // File inputFile = new File("JavaFile.txt");
        // File outputFile = new File("JavaOutputFile.txt");
        // FileReader fileReader = new FileReader(inputFile);
        // Here it ends

        MyScanner sc = new MyScanner();
        // MyScanner sc = new MyScanner(fileReader);

        out = new PrintWriter(new BufferedOutputStream(System.out)); // Output to console
        // out = new PrintWriter(new PrintStream(outputFile)); // Output to file

        getAns(sc);

        out.close();
    }

    /*
     *Don't use builtin function (Math.ceil)
     */

    static final long MOD = (long) (1e9 + 7);

    private static void getAns(MyScanner sc) {
        int n = sc.ni(), d = sc.ni();
        int one = 2 * d + 1;
        out.println((int) Math.ceil((double) n / one));
    }

    static class MyScanner {

        BufferedReader br;
        StringTokenizer st;

        MyScanner(FileReader fileReader) {
            br = new BufferedReader(fileReader);
        }

        MyScanner() {
            br = new BufferedReader(new InputStreamReader(System.in));
        }

        String nn() {
            while (st == null || !st.hasMoreElements()) {
                try {
                    st = new StringTokenizer(br.readLine());
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
            return st.nextToken();
        }

        String nextLine() {
            String ans = "";
            try {
                ans = br.readLine();
            } catch (IOException ex) {
                ex.printStackTrace();
            }
            return ans;
        }

        char nc() {
            return nn().charAt(0);
        }

        int ni() {
            return Integer.parseInt(nn());
        }

        long nl() {
            return Long.parseLong(nn());
        }

        double nd() {
            return Double.parseDouble(nn());
        }

        int[] niArr0(int n) {
            int[] ar = new int[n];
            for (int i = 0; i < n; i++) {
                ar[i] = ni();
            }
            return ar;
        }

        int[] niArr1(int n) {
            int[] ar = new int[n + 1];
            for (int i = 1; i <= n; i++) {
                ar[i] = ni();
            }
            return ar;
        }

        long[] nlArr0(int n) {
            long[] ar = new long[n];
            for (int i = 0; i < n; i++) {
                ar[i] = nl();
            }
            return ar;
        }

        long[] nlArr1(int n) {
            long[] ar = new long[n + 1];
            for (int i = 1; i <= n; i++) {
                ar[i] = nl();
            }
            return ar;
        }
    }
} import java.io.*; io.* java import java.util.*; util.* java class Main {

    private static PrintWriter out;

    private static <T> void mprintln(T... ar) {
        for (T i : ar) {
            out.print(i + " ");
        }
        out.println();
    }

    public static void main(String[] args) throws FileNotFoundException {

        // Input from file
        // File inputFile = new File("JavaFile.txt");
        // File outputFile = new File("JavaOutputFile.txt");
        // FileReader fileReader = new FileReader(inputFile);
        // Here it ends

        MyScanner sc = new MyScanner();
        // MyScanner sc = new MyScanner(fileReader);

        out = new PrintWriter(new BufferedOutputStream(System.out)); // Output to console
        // out = new PrintWriter(new PrintStream(outputFile)); // Output to file

        getAns(sc);

        out.close();
    }

    /*
     *Don't use builtin function (Math.ceil)
     */

    static final long MOD = (long) (1e9 + 7);

    private static void getAns(MyScanner sc) {
        int n = sc.ni(), d = sc.ni();
        int one = 2 * d + 1;
        out.println((int) Math.ceil((double) n / one));
    }

    static class MyScanner {

        BufferedReader br;
        StringTokenizer st;

        MyScanner(FileReader fileReader) {
            br = new BufferedReader(fileReader);
        }

        MyScanner() {
            br = new BufferedReader(new InputStreamReader(System.in));
        }

        String nn() {
            while (st == null || !st.hasMoreElements()) {
                try {
                    st = new StringTokenizer(br.readLine());
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
            return st.nextToken();
        }

        String nextLine() {
            String ans = "";
            try {
                ans = br.readLine();
            } catch (IOException ex) {
                ex.printStackTrace();
            }
            return ans;
        }

        char nc() {
            return nn().charAt(0);
        }

        int ni() {
            return Integer.parseInt(nn());
        }

        long nl() {
            return Long.parseLong(nn());
        }

        double nd() {
            return Double.parseDouble(nn());
        }

        int[] niArr0(int n) {
            int[] ar = new int[n];
            for (int i = 0; i < n; i++) {
                ar[i] = ni();
            }
            return ar;
        }

        int[] niArr1(int n) {
            int[] ar = new int[n + 1];
            for (int i = 1; i <= n; i++) {
                ar[i] = ni();
            }
            return ar;
        }

        long[] nlArr0(int n) {
            long[] ar = new long[n];
            for (int i = 0; i < n; i++) {
                ar[i] = nl();
            }
            return ar;
        }

        long[] nlArr1(int n) {
            long[] ar = new long[n + 1];
            for (int i = 1; i <= n; i++) {
                ar[i] = nl();
            }
            return ar;
        }
    }
} Main private static PrintWriter out; out static final long MOD = (long) (1e9 + 7); MOD (long) (1e9 + 7) 1e9 7 private static <T> void mprintln(T... ar) {
        for (T i : ar) {
            out.print(i + " ");
        }
        out.println();
    } mprintln {
        for (T i : ar) {
            out.print(i + " ");
        }
        out.println();
    } for (T i : ar) {
            out.print(i + " ");
        } T i ar {
            out.print(i + " ");
        } {
            out.print(i + " ");
        } out.print(i + " ") out.print out i + " " i " " out.println() out.println out T... ar ar public static void main(String[] args) throws FileNotFoundException {

        // Input from file
        // File inputFile = new File("JavaFile.txt");
        // File outputFile = new File("JavaOutputFile.txt");
        // FileReader fileReader = new FileReader(inputFile);
        // Here it ends

        MyScanner sc = new MyScanner();
        // MyScanner sc = new MyScanner(fileReader);

        out = new PrintWriter(new BufferedOutputStream(System.out)); // Output to console
        // out = new PrintWriter(new PrintStream(outputFile)); // Output to file

        getAns(sc);

        out.close();
    } main {

        // Input from file
        // File inputFile = new File("JavaFile.txt");
        // File outputFile = new File("JavaOutputFile.txt");
        // FileReader fileReader = new FileReader(inputFile);
        // Here it ends

        MyScanner sc = new MyScanner();
        // MyScanner sc = new MyScanner(fileReader);

        out = new PrintWriter(new BufferedOutputStream(System.out)); // Output to console
        // out = new PrintWriter(new PrintStream(outputFile)); // Output to file

        getAns(sc);

        out.close();
    } MyScanner sc = new MyScanner(); sc new MyScanner() out = new PrintWriter(new BufferedOutputStream(System.out)) out new PrintWriter(new BufferedOutputStream(System.out)) getAns(sc) getAns sc out.close() out.close out String[] args args private static void getAns(MyScanner sc) {
        int n = sc.ni(), d = sc.ni();
        int one = 2 * d + 1;
        out.println((int) Math.ceil((double) n / one));
    } getAns {
        int n = sc.ni(), d = sc.ni();
        int one = 2 * d + 1;
        out.println((int) Math.ceil((double) n / one));
    } int n = sc.ni() n sc.ni() sc.ni sc d = sc.ni(); d sc.ni() sc.ni sc int one = 2 * d + 1; one 2 * d + 1 2 * d 2 d 1 out.println((int) Math.ceil((double) n / one)) out.println out (int) Math.ceil((double) n / one) Math.ceil Math (double) n / one (double) n one MyScanner sc sc static class MyScanner {

        BufferedReader br;
        StringTokenizer st;

        MyScanner(FileReader fileReader) {
            br = new BufferedReader(fileReader);
        }

        MyScanner() {
            br = new BufferedReader(new InputStreamReader(System.in));
        }

        String nn() {
            while (st == null || !st.hasMoreElements()) {
                try {
                    st = new StringTokenizer(br.readLine());
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
            return st.nextToken();
        }

        String nextLine() {
            String ans = "";
            try {
                ans = br.readLine();
            } catch (IOException ex) {
                ex.printStackTrace();
            }
            return ans;
        }

        char nc() {
            return nn().charAt(0);
        }

        int ni() {
            return Integer.parseInt(nn());
        }

        long nl() {
            return Long.parseLong(nn());
        }

        double nd() {
            return Double.parseDouble(nn());
        }

        int[] niArr0(int n) {
            int[] ar = new int[n];
            for (int i = 0; i < n; i++) {
                ar[i] = ni();
            }
            return ar;
        }

        int[] niArr1(int n) {
            int[] ar = new int[n + 1];
            for (int i = 1; i <= n; i++) {
                ar[i] = ni();
            }
            return ar;
        }

        long[] nlArr0(int n) {
            long[] ar = new long[n];
            for (int i = 0; i < n; i++) {
                ar[i] = nl();
            }
            return ar;
        }

        long[] nlArr1(int n) {
            long[] ar = new long[n + 1];
            for (int i = 1; i <= n; i++) {
                ar[i] = nl();
            }
            return ar;
        }
    } MyScanner BufferedReader br; br StringTokenizer st; st MyScanner(FileReader fileReader) {
            br = new BufferedReader(fileReader);
        } MyScanner {
            br = new BufferedReader(fileReader);
        } br = new BufferedReader(fileReader) br new BufferedReader(fileReader) FileReader fileReader fileReader MyScanner() {
            br = new BufferedReader(new InputStreamReader(System.in));
        } MyScanner {
            br = new BufferedReader(new InputStreamReader(System.in));
        } br = new BufferedReader(new InputStreamReader(System.in)) br new BufferedReader(new InputStreamReader(System.in)) String nn() {
            while (st == null || !st.hasMoreElements()) {
                try {
                    st = new StringTokenizer(br.readLine());
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
            return st.nextToken();
        } nn {
            while (st == null || !st.hasMoreElements()) {
                try {
                    st = new StringTokenizer(br.readLine());
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
            return st.nextToken();
        } while (st == null || !st.hasMoreElements()) {
                try {
                    st = new StringTokenizer(br.readLine());
                } catch (IOException e) {
                    e.printStackTrace();
                }
            } st == null || !st.hasMoreElements() st == null st null !st.hasMoreElements() st.hasMoreElements() st.hasMoreElements st {
                try {
                    st = new StringTokenizer(br.readLine());
                } catch (IOException e) {
                    e.printStackTrace();
                }
            } try {
                    st = new StringTokenizer(br.readLine());
                } catch (IOException e) {
                    e.printStackTrace();
                } catch (IOException e) {
                    e.printStackTrace();
                } IOException e {
                    e.printStackTrace();
                } e.printStackTrace() e.printStackTrace e {
                    st = new StringTokenizer(br.readLine());
                } st = new StringTokenizer(br.readLine()) st new StringTokenizer(br.readLine()) return st.nextToken(); st.nextToken() st.nextToken st String nextLine() {
            String ans = "";
            try {
                ans = br.readLine();
            } catch (IOException ex) {
                ex.printStackTrace();
            }
            return ans;
        } nextLine {
            String ans = "";
            try {
                ans = br.readLine();
            } catch (IOException ex) {
                ex.printStackTrace();
            }
            return ans;
        } String ans = ""; ans "" try {
                ans = br.readLine();
            } catch (IOException ex) {
                ex.printStackTrace();
            } catch (IOException ex) {
                ex.printStackTrace();
            } IOException ex {
                ex.printStackTrace();
            } ex.printStackTrace() ex.printStackTrace ex {
                ans = br.readLine();
            } ans = br.readLine() ans br.readLine() br.readLine br return ans; ans char nc() {
            return nn().charAt(0);
        } nc {
            return nn().charAt(0);
        } return nn().charAt(0); nn().charAt(0) nn().charAt nn() nn 0 int ni() {
            return Integer.parseInt(nn());
        } ni {
            return Integer.parseInt(nn());
        } return Integer.parseInt(nn()); Integer.parseInt(nn()) Integer.parseInt Integer nn() nn long nl() {
            return Long.parseLong(nn());
        } nl {
            return Long.parseLong(nn());
        } return Long.parseLong(nn()); Long.parseLong(nn()) Long.parseLong Long nn() nn double nd() {
            return Double.parseDouble(nn());
        } nd {
            return Double.parseDouble(nn());
        } return Double.parseDouble(nn()); Double.parseDouble(nn()) Double.parseDouble Double nn() nn int[] niArr0(int n) {
            int[] ar = new int[n];
            for (int i = 0; i < n; i++) {
                ar[i] = ni();
            }
            return ar;
        } niArr0 {
            int[] ar = new int[n];
            for (int i = 0; i < n; i++) {
                ar[i] = ni();
            }
            return ar;
        } int[] ar = new int[n]; ar new int[n] n for (int i = 0; i < n; i++) {
                ar[i] = ni();
            } int i = 0; int i = 0; i 0 i < n i n i++ i++ i {
                ar[i] = ni();
            } {
                ar[i] = ni();
            } ar[i] = ni() ar[i] ar i ni() ni return ar; ar int n n int[] niArr1(int n) {
            int[] ar = new int[n + 1];
            for (int i = 1; i <= n; i++) {
                ar[i] = ni();
            }
            return ar;
        } niArr1 {
            int[] ar = new int[n + 1];
            for (int i = 1; i <= n; i++) {
                ar[i] = ni();
            }
            return ar;
        } int[] ar = new int[n + 1]; ar new int[n + 1] n + 1 n 1 for (int i = 1; i <= n; i++) {
                ar[i] = ni();
            } int i = 1; int i = 1; i 1 i <= n i n i++ i++ i {
                ar[i] = ni();
            } {
                ar[i] = ni();
            } ar[i] = ni() ar[i] ar i ni() ni return ar; ar int n n long[] nlArr0(int n) {
            long[] ar = new long[n];
            for (int i = 0; i < n; i++) {
                ar[i] = nl();
            }
            return ar;
        } nlArr0 {
            long[] ar = new long[n];
            for (int i = 0; i < n; i++) {
                ar[i] = nl();
            }
            return ar;
        } long[] ar = new long[n]; ar new long[n] n for (int i = 0; i < n; i++) {
                ar[i] = nl();
            } int i = 0; int i = 0; i 0 i < n i n i++ i++ i {
                ar[i] = nl();
            } {
                ar[i] = nl();
            } ar[i] = nl() ar[i] ar i nl() nl return ar; ar int n n long[] nlArr1(int n) {
            long[] ar = new long[n + 1];
            for (int i = 1; i <= n; i++) {
                ar[i] = nl();
            }
            return ar;
        } nlArr1 {
            long[] ar = new long[n + 1];
            for (int i = 1; i <= n; i++) {
                ar[i] = nl();
            }
            return ar;
        } long[] ar = new long[n + 1]; ar new long[n + 1] n + 1 n 1 for (int i = 1; i <= n; i++) {
                ar[i] = nl();
            } int i = 1; int i = 1; i 1 i <= n i n i++ i++ i {
                ar[i] = nl();
            } {
                ar[i] = nl();
            } ar[i] = nl() ar[i] ar i nl() nl return ar; ar int n n
import java.io.*;
import java.util.*;

class Main {

    private static PrintWriter out;

    private static <T> void mprintln(T... ar) {
        for (T i : ar) {
            out.print(i + " ");
        }
        out.println();
    }

    public static void main(String[] args) throws FileNotFoundException {

        // Input from file
        // File inputFile = new File("JavaFile.txt");
        // File outputFile = new File("JavaOutputFile.txt");
        // FileReader fileReader = new FileReader(inputFile);
        // Here it ends

        MyScanner sc = new MyScanner();
        // MyScanner sc = new MyScanner(fileReader);

        out = new PrintWriter(new BufferedOutputStream(System.out)); // Output to console
        // out = new PrintWriter(new PrintStream(outputFile)); // Output to file

        getAns(sc);

        out.close();
    }

    /*
     *Don't use builtin function (Math.ceil)
     */

    static final long MOD = (long) (1e9 + 7);

    private static void getAns(MyScanner sc) {
        int n = sc.ni(), d = sc.ni();
        int one = 2 * d + 1;
        out.println((int) Math.ceil((double) n / one));
    }

    static class MyScanner {

        BufferedReader br;
        StringTokenizer st;

        MyScanner(FileReader fileReader) {
            br = new BufferedReader(fileReader);
        }

        MyScanner() {
            br = new BufferedReader(new InputStreamReader(System.in));
        }

        String nn() {
            while (st == null || !st.hasMoreElements()) {
                try {
                    st = new StringTokenizer(br.readLine());
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
            return st.nextToken();
        }

        String nextLine() {
            String ans = "";
            try {
                ans = br.readLine();
            } catch (IOException ex) {
                ex.printStackTrace();
            }
            return ans;
        }

        char nc() {
            return nn().charAt(0);
        }

        int ni() {
            return Integer.parseInt(nn());
        }

        long nl() {
            return Long.parseLong(nn());
        }

        double nd() {
            return Double.parseDouble(nn());
        }

        int[] niArr0(int n) {
            int[] ar = new int[n];
            for (int i = 0; i < n; i++) {
                ar[i] = ni();
            }
            return ar;
        }

        int[] niArr1(int n) {
            int[] ar = new int[n + 1];
            for (int i = 1; i <= n; i++) {
                ar[i] = ni();
            }
            return ar;
        }

        long[] nlArr0(int n) {
            long[] ar = new long[n];
            for (int i = 0; i < n; i++) {
                ar[i] = nl();
            }
            return ar;
        }

        long[] nlArr1(int n) {
            long[] ar = new long[n + 1];
            for (int i = 1; i <= n; i++) {
                ar[i] = nl();
            }
            return ar;
        }
    }
}