#include <iostream>
#include <vector>
using namespace std;
int main()
{
    int N = 45;
    int M;
    cin >> M;

    vector <long long> a(N+2);
    a[0] = 2, a[1] = 2;



    for (int i = 0; i < N; i++)
    {
        a[i+2] = (a[i+1] + a[i]);
    }



       //Fibonacci sequence

       // F0 = 2;
       // F1 = 2;
       // Fn = Fn-1 + Fn-2


    cout << a[M-1];
    return 0;
}