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();
	println!(
		"{}",
		if k == 0 {
			PI.powi(2) / 6.
		} else {
			(1..=k).map(|i| 1. / i as f64).sum::<f64>() / k as f64
		}
	)
}