fn main() {
    let x: f32 = {
        let mut line: String = String::new();
        std::io::stdin().read_line(&mut line).unwrap();
        line.trim().parse().unwrap()
    };

    let mut ANS:f32=0.0000000005;

    for i in 1..1200000000{
        ANS+=1.0/((i as f32 +x)*(i as f32 +x));
    }

    println!("{}",ANS);


}