#include <iostream>
#include <fstream>
using namespace std;

int main()
{
    ifstream f("nunta.in");
    ofstream g("nunta.out");
    int N,a,b,c,i;
    a=1;
    b=2;
    f>>N;
    for(i=2;i<N;i++){
        c=a+b;
        a=b;
        b=c;
    }
    g<<c;
}