#include <iostream>
#include <vector>

std::vector<int> m(32);
int f(int n)
{
    if (n == 0) return 0;
    if (n == 1) return 1;
    if (n == 2) return 2;
    if (m[n] == -1) m[n] = f(n - 1) + f(n - 2) + f(n - 3);
    return m[n];
}

int main()
{
    int N;
    std::cin >> N;

    std::cout << f(N) << std::endl;

    system("pause");
    return 0;
}