fn read<T: std::str::FromStr>() -> T {
    let mut s = String::new();
    std::io::stdin().read_line(&mut s).ok();
    s.trim().parse().ok().unwrap()
}

fn main() {
    let num: f64 = read();

    println!("{}", 2 * num * std::f64::consts::PI);
}
