fn main() {
    let mut n = String::new();
    std::io::stdin().read_line(&mut n).ok();
    let n: isize = n.trim().parse().unwrap();

    let mut ans = 1.0;
    for i in 2..=n {
        if i == 2 { ans += 1.0 }
        else {
            let mut x = 0;
            for j in 1..i {
                for k in j + 1..i {
                    x += j * k;
                }
            }
            ans += 2.0 / i as f64 + 2.0 * x as f64 / i as f64 / (i - 1) as f64;
        }
    }
    
    println!("{}", ans);
}