import java.util.*;
import java.util.stream.IntStream;
import java.util.stream.Stream;

class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int h = sc.nextInt();
        System.out.println(h*2*Math.PI);
    }
} import java.util.*; util.* java import java.util.stream.IntStream; IntStream java.util.stream import java.util.stream.Stream; Stream java.util.stream class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int h = sc.nextInt();
        System.out.println(h*2*Math.PI);
    }
} Main public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int h = sc.nextInt();
        System.out.println(h*2*Math.PI);
    } main {
        Scanner sc = new Scanner(System.in);
        int h = sc.nextInt();
        System.out.println(h*2*Math.PI);
    } Scanner sc = new Scanner(System.in); sc new Scanner(System.in) int h = sc.nextInt(); h sc.nextInt() sc.nextInt sc System.out.println(h*2*Math.PI) System.out.println System.out System System.out h*2*Math.PI h*2*Math.PI h 2 Math.PI Math Math.PI String[] args args
import java.util.*;
import java.util.stream.IntStream;
import java.util.stream.Stream;

class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int h = sc.nextInt();
        System.out.println(h*2*Math.PI);
    }
}