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.BufferedReader;
import java.io.File;
import java.io.FileNotFoundException;
import java.io.FileReader;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.io.PrintWriter;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collections;
import java.util.HashSet;
import java.util.Iterator;
import java.util.LinkedHashSet;
import java.util.LinkedList;
import java.util.PriorityQueue;
import java.util.Queue;
import java.util.Scanner;
import java.util.Set;
import java.util.Stack;
import java.util.StringTokenizer;
import java.util.TreeSet;
public class Main {
	//YOU CAN DO IT ....
	//Do what you love to do and give it your very best. Whether it's business or baseball, or the theater, or any field.
	//If you don't love what you're doing and you can't give it your best, get out of it. Life is too short. You'll be an old man before you know it.
	//Don't aim for success if you want it; just do what you love and believe in, and it will come naturally.
static int n; static int m; static int ans;static long count; static int flag;
static int a[];static int seg[];static boolean isPrime[];static boolean visited[] ;static int dp[][];
static ArrayList<pair> s[];static ArrayList<Integer>list;
static class pair{String first;int second;public pair(String x,int y){first=x;second=y;}}
static void seive(){isPrime[0] = isPrime[1] = true;for (int i = 2; i*i <= 2000000; i++) {if(isPrime[(int) i]) { long j=0; for ( j = i*i; j <= 2000000; j += i)isPrime[(int) j] = true;}}}
static int V;
static ArrayList<Edge>[] adjList;

static int[] dijkstra(int S, int T)
{
	long[] dist = new long[V];
	int[] p = new int[V];
	Arrays.fill(dist, (long)1e16);
	Arrays.fill(p, -1);
	dist[S] = 0;
	
	PriorityQueue<Edge> pq = new PriorityQueue<Edge>();
	pq.add(new Edge(S, 0));
	while(!pq.isEmpty())
	{
		Edge cur = pq.remove();//awal node
		if(cur.to == T)
			return p;
		if(cur.cost > dist[cur.to])
			continue;
		for(Edge nxt: adjList[cur.to])
			if(nxt.cost + cur.cost < dist[nxt.to])
			{
				pq.add(new Edge(nxt.to, dist[nxt.to] = nxt.cost + cur.cost));
				p[nxt.to] = cur.to;
			}
	}
	return null;
}
    public static void main(String[] args) throws FileNotFoundException {
    	Scanner sc = new Scanner(System.in);


		int n = sc.nextInt();
		System.out.print((n/3)*2+(n % 3 != 0 ? 1 : 0));
		
			
	}
   
	}
    	
    	
    	
    	
    	
    	
    	
    	
    	
    	
    	
    	
    	
    	
    	
    	
    

class Edge implements Comparable<Edge>
{
	int to;//second node
	long cost;//weight
	
	Edge(int x, long y)
	{
		to = x;
		cost = y;
	}
	
	public int compareTo(Edge e)
	{
		if(cost != e.cost)
			return cost > e.cost ? 1 : -1;
		return 0;		
	}
}
   class couple implements Comparable<couple>
   { String x;
   int y;
     public couple(String m,int f) {
    	 x=m;
    	 y=f;
     }
	public int compareTo(couple o) {
		
		 
		return y-o.y;
	}
	static class Scanner 
	{
		StringTokenizer st;
		BufferedReader br;

		public Scanner(InputStream s){    br = new BufferedReader(new InputStreamReader(s));}

		public Scanner(FileReader r){    br = new BufferedReader(r);}

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

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

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

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

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

		public boolean ready() throws IOException {return br.ready();}


	}
} 