#include<iostream>
using namespace std;
 
int main(){
	long long a,b,c,x,n,i;
	cin>>n;
	a = 1;
	b = 2;
	c = 4;
	if(n == 1){
		cout<<1<<endl;
	}else if(n == 2){
		cout<<2<<endl;
	}else if(n == 3){
		cout<<4<<endl;
	}else{
		for(i = 4;i <= n;i++){
			x = a + b + c;
			a = b;
			b = c;
			c = x;
		}
		cout<<x<<endl;
	}
	
	return 0;
}
/**************************************************************
	Problem: 1367
	User: admin
	Language: C++
	Result: Accepted
	Time:21 ms
	Memory:2072 kb
****************************************************************/