#include <iostream>
using namespace std;

int num(int n);

int main()
{
  string s;
  cin >> s;
  int n = stoi(s)
  cout << num(n) <<endl;
}

int num(int n){
  if(n<0)return 0;
  return num(n-1)+num(n-2)+num(n-3);
}