#include <iostream>
#include <vector>
#include <string>
#include <algorithm>
#include <cstdio>
#include <cstring>
#include <cmath>
using namespace std;
using ll = long long;

int main() {
    int n;
    cin >> n;

    ll p[51][2] = {};
    p[0][0] = 1;
    p[1][0] = 1;
    for (int i = 2; i <= n; i++) {
        p[i][0] = p[i - 1][0] + p[i - 1][1];
        p[i][1] = p[i - 2][0] + p[i - 2][1];
    }

    cout << p[n][0] + p[n][1] << endl;

    return 0;
}