#include<stdio.h>
#include<math.h>
int main()
{	
	int n;
	double f,a,b;
	scanf("%d",&n);
	a=pow(((1+pow(5,0.5))/2.0),n);
	b=pow(((1-pow(5,0.5))/2.0),n);
	f=((a-b)/1.0*pow(5,0.5));
	printf("%.2f",f);
	return 0;	
}