Little Artem got n stones on his birthday and now wants to give some of them to Masha. He knows that Masha cares more about the fact of receiving the present, rather than the value of that present, so he wants to give her stones as many times as possible. However, Masha remembers the last present she received, so Artem can't give her the same number of stones twice in a row. For example, he can give her 3 stones, then 1 stone, then again 3 stones, but he can't give her 3 stones and then again 3 stones right after that.

How many times can Artem give presents to Masha?

Input

The only line of the input contains a single integer n (1 ≤ n ≤ 109) — number of stones Artem received on his birthday.

Output

Print the maximum possible number of times Artem can give presents to Masha.

Examples

Input

1


Output

1


Input

2


Output

1


Input

3


Output

2


Input

4


Output

3

Note

In the first sample, Artem can only give 1 stone to Masha.

In the second sample, Atrem can give Masha 1 or 2 stones, though he can't give her 1 stone two times.

In the third sample, Atrem can first give Masha 2 stones, a then 1 more stone.

In the fourth sample, Atrem can first give Masha 1 stone, then 2 stones, and finally 1 stone again.
import java.io.BufferedOutputStream;
import java.io.BufferedReader;
import java.io.File;
import java.io.FileNotFoundException;
import java.io.FileOutputStream;
import java.io.FileReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.OutputStream;
import java.io.PrintStream;
import java.util.StringTokenizer;

public class RussianCodeCupC {

    private static Scanner in;
    private static Output out;
    static boolean isFile = false;

    public static void solve() throws Exception {
        long n = in.nextLong();
        long twos = n / 3;
        long left = n % 3;
        out.println(twos * 2 + (left == 0 ? 0 : 1));
    }

    static Exception exception;

    public static void main(String[] args) throws Exception {
        Thread thread = new Thread(null, new Runnable() {
            @Override
            public void run() {
                try {
                    initReaderWriter();
                    solve();
                    out.close();
                } catch (Exception ex) {
                    exception = ex;
                }
            }
        }, "", 1 << 26);
        thread.start();
        thread.join();

        if (exception != null) {
            throw exception;
        }
    }


    private static void initReaderWriter() throws Exception {
        if (isFile) {
            in = new Scanner("input.txt");
            out = new Output(new File("output.txt"));
        } else {
            in = new Scanner();
            out = new Output(System.out);
        }
    }

    private static boolean log = false;

    public static void log(String msg) {
        if (log) {
            out.println(msg);
            out.flush();
        }
    }

    private static class Scanner {

        StringTokenizer st = null;
        BufferedReader bf;

        public Scanner() {
            bf = new BufferedReader(new InputStreamReader(System.in));
        }

        public Scanner(String fileName) throws FileNotFoundException {
            bf = new BufferedReader(new FileReader(fileName));
        }

        public String next() throws IOException {
            while (st == null || !st.hasMoreTokens())
                st = new StringTokenizer(bf.readLine());
            return st.nextToken();
        }

        public String nextLine() throws IOException {
            return bf.readLine();
        }

        public int nextInt() throws IOException {
            return Integer.parseInt(next());
        }

        public long nextLong() throws IOException {
            return Long.parseLong(next());
        }

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

    private static class Output extends PrintStream {

        public Output(OutputStream out) {
            super(new BufferedOutputStream(out));
        }

        public Output(File file) throws FileNotFoundException {
            super(new BufferedOutputStream(new FileOutputStream(file)));
        }
    }

    private static void printMemory() {
        Runtime runtime = Runtime.getRuntime();

        long maxMemory = runtime.maxMemory();
        long allocatedMemory = runtime.totalMemory();
        long freeMemory = runtime.freeMemory();

        long div = 1L * 1024 * 1024;
        // long div = 1L ;

        System.out.println("used memory[mgb]: " + (runtime.totalMemory() - runtime.freeMemory()) / div);

        System.out.println();
    }

    static long appCurrentTime = System.currentTimeMillis();
    static long appCurrentTimeNano = System.nanoTime();

    private static void printCurrentTime() {
        out.flush();
        System.out.println("Time: " + (System.currentTimeMillis() - appCurrentTime));
        System.out.println("Time Nano: " + (System.nanoTime() - appCurrentTimeNano));
    }
}
