import java.util.*;
import java.lang.*;
import java.io.*;
 
class Main
{
    public static void main (String[] args) throws java.lang.Exception
    {
        Scanner in = new Scanner(System.in);
        long x1, x2, x3, 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);
    }
}

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NA==

4