import java.util.*;

class Main {
    public static void main (String[] args) {
        Scanner sc = new Scanner(System.in);
        int R = sc.nextInt();
        System.out.println(6.28 * R);

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

    }
} Main public static void main (String[] args) {
        Scanner sc = new Scanner(System.in);
        int R = sc.nextInt();
        System.out.println(6.28 * R);

    } main {
        Scanner sc = new Scanner(System.in);
        int R = sc.nextInt();
        System.out.println(6.28 * R);

    } Scanner sc = new Scanner(System.in); sc new Scanner(System.in) int R = sc.nextInt(); R sc.nextInt() sc.nextInt sc System.out.println(6.28 * R) System.out.println System.out System System.out 6.28 * R 6.28 R String[] args args
import java.util.*;

class Main {
    public static void main (String[] args) {
        Scanner sc = new Scanner(System.in);
        int R = sc.nextInt();
        System.out.println(6.28 * R);

    }
}