#include<bits/stdc++.h>
using namespace std;
int main(){
	long long n;
	double a=(1+sqrt(5))/2;
	double b=(1-sqrt(5))/2;
	double F;
	cin>>n;
	F=(pow (a,n)-pow (b,n))/sqrt(5);
	printf("%.2lf",F);
	return 0;
}