/*------------------------------------------------------------------------------
 * problem019.c: Counting (6.6.3)
**----------------------------------------------------------------------------*/

/* Copyright (c) 2009 Peter Bui. All Rights Reserved.
 *
 * Peter Bui <pbui@cse.nd.edu>
 *
**------------------------------------------------------------------------------
 * Includes
**----------------------------------------------------------------------------*/

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

/*------------------------------------------------------------------------------
 * Constants 
**----------------------------------------------------------------------------*/

#define MAX_INT	    100

/*------------------------------------------------------------------------------
 * Type Definitions
**----------------------------------------------------------------------------*/


/*------------------------------------------------------------------------------
 * Globals
**----------------------------------------------------------------------------*/

static double Counts[MAX_INT + 1] = { 0, 2, 5, 13 };

/*------------------------------------------------------------------------------
 * Functions
**----------------------------------------------------------------------------*/

/* Count(1) = 2
 * Count(2) = 5
 * Count(3) = 13
 * Count(n) = 2 * Count(n - 1) + Count(n - 2) + Count(n - 3) */

static double
count(int n)
{
    if (n > 3 && !Counts[n])
	Counts[n] = 2*count(n - 1) + count(n - 2) +  count(n - 3);

    return (Counts[n]);
}

/*------------------------------------------------------------------------------
 * Main Execution
**----------------------------------------------------------------------------*/

int
main(int argc, char* argv[]) 
{
    int n;

    while (scanf("%d", &n) != EOF)
	printf("%.lf\n", count(n));

    return (EXIT_SUCCESS);
}

/*------------------------------------------------------------------------------
 * vim: sts=4 sw=4 ts=8 ft=cpp
**----------------------------------------------------------------------------*/