#include <iostream>
#include <fstream>
#define LL long long int
using namespace std;

LL N;

int main(){
    ifstream in("nunta.in");
    ofstream out("nunta.out");
    in >> N;

    LL f=1,l=2,i,aux;
    for (i=3; i<=N; i++){
        aux=l;
        l=l+f;
        f=aux;
    }

    out << l;
    return 0;
}