#include <vector>
#include <iostream>
using namespace std;

static vector<int> louti;
int zoulouti1(int n)
{
    if(n <= louti.size()) // 已经计算过的楼梯级，直接返回vector里面的值即可
        return louti[n - 1];
    int i = louti.size();
    while(i < n) {
        // f(n) = f(n - 1) + f(n - 2) + f(n - 3)
        int zoufa = louti[i - 1] + louti[i - 2] + louti[i - 3];
        louti.push_back(zoufa);
        ++i;
    }
    return louti.back();
}
void init_louti()
{
    // 只有1、2、3级楼梯，对应的走法
    louti.push_back(1);
    louti.push_back(2);
    louti.push_back(4);
}
int main()
{
    int n = 0;
    cin >> n;
    init_louti();
    cout <<  zoulouti1(n) << endl;
}