#include<iostream>
#include<iomanip>
using namespace std;
int main(){
    int n;
    cin>>n;
    double sum=1.0;
    for(double i=2.0;i<=n;i++){
        sum+=2/i+(3*i*i-7*i+2)/12;
    }
    cout<<fixed<<setprecision(10)<<sum<<endl;
    return 0;
}