#include <iostream>
#include <cmath>
#include <fstream>

using namespace std;
ifstream f("nunta.in");
ofstream g("nunta.out");

int main()
{
    int N,i;
    long double fib;
    f>>N;
    fib=(pow(((1+sqrt(5))/2),N)-pow(((1-sqrt(5))/2),N))/sqrt(5);
    g<<fib;

    return 0;
}