#include <stdio.h>
#include <math.h>

double f_bizet(int n) {
  return (1.0/sqrt(5.0) * ( pow((1.0+sqrt(5.0))/2.0,(double) n)
			    -pow((1.0-sqrt(5.0))/2.0,(double) n)));
}

int main() {
  int n;

  printf("n: "); scanf("%d",&n);
  printf("%22.16e\n",f_bizet(n));
}