#include<iostream>
using namespace std;
int lo(int m)
{
	if(m==1)
	return 1;
	else if(m==2)
	return 2;
	else if(m==3)
	return 4;
	else
	return lo(m-1)+lo(m-2)+lo(m-3);
}
int main()
{
int m;
cin>>m;
cout<<lo(m);	
}