#include<bits/stdc++.h>
using namespace std;
double n,a[31],b[31];
double ans;
int main(){
    cin>>n;
    a[1]=1;
    b[1]=1;
    a[2]=1;
    b[2]=2;
    ans+=1.5;
    for(int i=3;i<=n;i++){
    	a[i]=a[i-1]+a[i-2];
    	b[i]=b[i-1]+b[i-2];
    	ans+=a[i]/b[i]*1.0;
	}
	printf("%.3f",ans);
    return 0;
}
/**************************************************************
	Problem: 1147
	User: chenjingqi
	Language: C++
	Result: Accepted
	Time:12 ms
	Memory:2072 kb
****************************************************************/