use std::f64::consts::PI;

fn main() {
	let mut s = String::new();
	std::io::stdin().read_line(&mut s).ok();
	let k: u8 = s.trim().parse().unwrap();
	if k == 0 {
		println!("{}", PI.powi(2) / 6.)
	} else {
		println!("{}", (1..=k).map(|i| 1. / i as f64).sum::<f64>() / k as f64)
	}
}