import java.lang.Math;
import java.util.Scanner;
 
class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int r = sc.nextInt();
        System.out.print(r*6.28);
    }
} import java.lang.Math; Math java.lang import java.util.Scanner; Scanner java.util class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int r = sc.nextInt();
        System.out.print(r*6.28);
    }
} Main public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int r = sc.nextInt();
        System.out.print(r*6.28);
    } main {
        Scanner sc = new Scanner(System.in);
        int r = sc.nextInt();
        System.out.print(r*6.28);
    } Scanner sc = new Scanner(System.in); sc new Scanner(System.in) int r = sc.nextInt(); r sc.nextInt() sc.nextInt sc System.out.print(r*6.28) System.out.print System.out System System.out r*6.28 r 6.28 String[] args args
import java.lang.Math;
import java.util.Scanner;
 
class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int r = sc.nextInt();
        System.out.print(r*6.28);
    }
}