#include <iostream>
#include <fstream>
using namespace std;
ifstream f("nunta.in");
ofstream g("nunta.out");
int main()
{
    long long unsigned n, a, b, p, c;
    f>>n;
    a=2;
    b=3;
    if(n<4){g<<n;
            f.close();
            g.close();
            return 0;}
    for(p=4; p<=n; p++)
    {
        c=a+b;
        a=b;
        b=c;
    }
    g<<c;
    f.close();
    g.close();
    return 0;
}