use std::io;
use std::f32::consts::PI;

fn main() {
    loop {
        let mut buf = String::new();
        let mut num: u32;
        io::stdin().read_line(&mut buf).expect("Failed to read line");
        num = buf.trim().parse().unwrap();
        num *= 2;
        println!("{}", (num as f32 * PI));
    }
}
