#include <bits/stdc++.h>
using namespace std;

int f(int n)
{
    int r;
    if (n==1)
    {
        r = 1;
    }
    else if(n==2)
    {
        r = 2;
    }
    else if(n==3)
    {
        r = 4;
    }
    else
    {
        r = f(n-1) + f(n-2) + f(n-3);
    }
    return r;
}
int main()
{

    int n;
    cin >> n;
    cout << f(n) <<endl;
   

    return 0;
}
/**************************************************************
	Problem: 1367
	User: mariooo
	Language: C++
	Result: Accepted
	Time:17 ms
	Memory:2072 kb
****************************************************************/