#include <iostream>
#include<fstream>
using namespace std;
ifstream f("nunta.in");
ofstream g("nunta.out");
int main()
{
    int N;
    f >> N;
    if(N < 3)
            g << 2;
        else
        {
            if(N == 3)
                g << 3;
            else
            {
                long long a, b, c;
                a = 2;
                b = 3;
                int i = N - 3;
                while(i > 0)
                {
                    c = a + b;
                    a = b;
                    b = c;
                    i--;
                }
                g << c;
            }
        }
        return 0;
    }