import java.util.*;
import java.util.Collections;

class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int R = sc.nextInt();

        //double PI = 3.1415926535897932;
        double PI = 3.14159265;
        double v = 2 *R* PI;

        System.out.println(v);

    }
} import java.util.*; util.* java import java.util.Collections; Collections java.util class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int R = sc.nextInt();

        //double PI = 3.1415926535897932;
        double PI = 3.14159265;
        double v = 2 *R* PI;

        System.out.println(v);

    }
} Main public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int R = sc.nextInt();

        //double PI = 3.1415926535897932;
        double PI = 3.14159265;
        double v = 2 *R* PI;

        System.out.println(v);

    } main {
        Scanner sc = new Scanner(System.in);
        int R = sc.nextInt();

        //double PI = 3.1415926535897932;
        double PI = 3.14159265;
        double v = 2 *R* PI;

        System.out.println(v);

    } Scanner sc = new Scanner(System.in); sc new Scanner(System.in) int R = sc.nextInt(); R sc.nextInt() sc.nextInt sc double PI = 3.14159265; PI 3.14159265 double v = 2 *R* PI; v 2 *R* PI 2 *R* PI 2 R PI System.out.println(v) System.out.println System.out System System.out v String[] args args
import java.util.*;
import java.util.Collections;

class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int R = sc.nextInt();

        //double PI = 3.1415926535897932;
        double PI = 3.14159265;
        double v = 2 *R* PI;

        System.out.println(v);

    }
}