#include <iostream>
#include <cstring>
#include <algorithm>
using namespace std;
int dfs(int n){
if(n == 1 || n == 2)
return n;
if(n == 3){
return 4;
}
return dfs(n - 1) + dfs(n - 2) + dfs(n - 3);
}
int main()
{
int N;
cin >> N;
cout << dfs(N);
}