#include <iostream>

using namespace std;

int fun(int n)
{
if (n == 0) return 0;
else if (n == 1) return 1;
else if (n == 2) return 2;
else if (n == 3) return 4;
else return fun(n - 1) + fun(n - 2) + fun(n - 3);
}

int main()
{
int x;
cin >> x;
cout<<fun(x)<<endl;

return 0;
}