#include <iostream.h>
#include <stdio.h>
#include <math.h>
int k;
int main()
{  freopen("kfib.in", "r", stdin);
   freopen("kfib.out", "w", stdout);
    cin>>k;
    cout<<( pow(1+sqrt(5),k)-pow(1-sqrt(5),k))/pow(2,k)/sqrt(5);
    
    return 0;
}