#include<bits/stdc++.h>
using namespace std;
int main(){
    double a[31],b[31],n,sum=0;
    cin>>n;
    for(int i=2;i<n-2;i++){
        a[0]=1;a[1]=2;
        b[0]=2;b[1]=3;
        a[i]=a[i-1]+a[i-2];
        b[i]=b[i-1]+b[i-2];
        sum+=b[i]/a[i];
    }
   cout<<fixed<<setprecision(4)<<sum+1.0*(2/1)+1.0*(3/2);
    return 0;
    
    
    
    
}