#include<iostream>
#include<cmath>
#include<algorithm>
#include<string>
#include<map>
using namespace std;
int sum(int n) {
	if (n == 1)return 1;
	if (n == 2)return 2;
	if (n == 3)return 4;
	return sum(n - 1) + sum(n - 2) + sum(n - 3);
}
int main() {
	int n;
	while (cin >> n) {
		cout << sum(n) << endl;
	}
	return 0;
}