912

#include <iostream>
#include <fstream>
#include <vector>

using namespace std;

int main() {
    ifstream fin("input.txt");
    ofstream fout("output.txt");

    int n;
    fin >> n;
    vector<int> dp(n + 3);
    dp[1] = 2;
    dp[2] = 4;
    dp[3] = 7;
    for (int i = 4; i <= n; i++)
        dp[i] = dp[i - 1] + dp[i - 2] + dp[i - 3];
    fout << dp[n] << endl;


    fin.close();
    fout.close();
    return 0;
}