#include<iostream>
#include<cstdio>
#include<cmath>
#include<algorithm> 
#include<string>
#include<cstring>
using namespace std;
int main()
{
    int n;
    while (cin >> n && n > 0) {
        int a[1000] = { 0 };
        a[0] = 1, a[1] = 2, a[2] = 4;
        for (int i = 3; i <= n; i++) {
            a[i] = a[i - 1] + a[i - 2] + a[i - 3];
        }

        cout << a[n-1] << endl;
    }
    return 0;
}