#include <bits/stdc++.h>

using namespace std;

int dp[25];

int n;

int main()
{
    cin >> n;

    dp[0] = 1;

    for(int i = 1; i <= n; i ++)
    {
        if(i >= 1 and i < 2)
            dp[i] += dp[i - 1];

        if(i >= 2 and i < 3)
            dp[i] += dp[i - 1] + dp[i - 2];

        if(i >= 3)
            dp[i] += dp[i - 1] + dp[i - 2] + dp[i - 3];
    }

    cout << dp[n];
}