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

class Ideone
{
	public static void main (String[] args) throws java.lang.Exception
	{
		Scanner in = new Scanner(System.in);
		long x1,x2,x3;
		long x4=0;
		long n=in.nextLong();
		x1=2;
		x2=4;
		x3=7;
		for (int i=3; i<n; i++) {
			x4=x1+x2+x3;
			x1=x2;
			x2=x3;
			x3=x4;    
		}
		if (n==1) 
			System.out.println(2); 
		else if (n==2) 
			System.out.println(4); 
		else if (n==3)
			System.out.println(7); 
		else 
			System.out.println(x4);
	}
}