import java.util.Scanner;

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

        int R = sc.nextInt();

        System.out.println(solve(R));

        sc.close();
    }

    static double solve(int R) {
        return 2 * Math.PI * R;
    }
} import java.util.Scanner; Scanner java.util public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);

        int R = sc.nextInt();

        System.out.println(solve(R));

        sc.close();
    }

    static double solve(int R) {
        return 2 * Math.PI * R;
    }
} Main public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);

        int R = sc.nextInt();

        System.out.println(solve(R));

        sc.close();
    } main {
        Scanner sc = new Scanner(System.in);

        int R = sc.nextInt();

        System.out.println(solve(R));

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

        int R = sc.nextInt();

        System.out.println(solve(R));

        sc.close();
    }

    static double solve(int R) {
        return 2 * Math.PI * R;
    }
} public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);

        int R = sc.nextInt();

        System.out.println(solve(R));

        sc.close();
    }

    static double solve(int R) {
        return 2 * Math.PI * R;
    }
} Main
import java.util.Scanner;

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

        int R = sc.nextInt();

        System.out.println(solve(R));

        sc.close();
    }

    static double solve(int R) {
        return 2 * Math.PI * R;
    }
}