#include<bits/stdc++.h>
using namespace std;
int main()
{
    int n;
    cin>>n;
    int a,b,c,x;
    a = 1;
    b = 2;
    c = 4;
    if(n==1)
    {
        cout<<1;
    }
    else if(n==2)
    {
        cout<<2;
    }
    else if(n==3)
    {
        cout<<4;
    }
    else
    {
        for(int i = 4;i<=n;i++)
        {
            x = a + b + c;
            a = b;
            b = c;
            c = x;
        }
        cout<<x;
    }
}
/**************************************************************
	Problem: 1367
	User: wuyichen
	Language: C++
	Result: Accepted
	Time:28 ms
	Memory:2072 kb
****************************************************************/