import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.util.Scanner;

public class Main {

    public static void main(String[] args) {
        final Scanner in = new Scanner(new BufferedReader(new InputStreamReader(System.in)));
        int r = in.nextInt();
        System.out.println(2.0 * Math.PI * r);
    }
} import java.io.BufferedReader; BufferedReader java.io import java.io.InputStreamReader; InputStreamReader java.io import java.util.Scanner; Scanner java.util public class Main {

    public static void main(String[] args) {
        final Scanner in = new Scanner(new BufferedReader(new InputStreamReader(System.in)));
        int r = in.nextInt();
        System.out.println(2.0 * Math.PI * r);
    }
} Main public static void main(String[] args) {
        final Scanner in = new Scanner(new BufferedReader(new InputStreamReader(System.in)));
        int r = in.nextInt();
        System.out.println(2.0 * Math.PI * r);
    } main {
        final Scanner in = new Scanner(new BufferedReader(new InputStreamReader(System.in)));
        int r = in.nextInt();
        System.out.println(2.0 * Math.PI * r);
    } final Scanner in = new Scanner(new BufferedReader(new InputStreamReader(System.in))); in new Scanner(new BufferedReader(new InputStreamReader(System.in))) int r = in.nextInt(); r in.nextInt() in.nextInt in System.out.println(2.0 * Math.PI * r) System.out.println System.out System System.out 2.0 * Math.PI * r 2.0 * Math.PI * r 2.0 Math.PI Math Math.PI r String[] args args public class Main {

    public static void main(String[] args) {
        final Scanner in = new Scanner(new BufferedReader(new InputStreamReader(System.in)));
        int r = in.nextInt();
        System.out.println(2.0 * Math.PI * r);
    }
} public class Main {

    public static void main(String[] args) {
        final Scanner in = new Scanner(new BufferedReader(new InputStreamReader(System.in)));
        int r = in.nextInt();
        System.out.println(2.0 * Math.PI * r);
    }
} Main
import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.util.Scanner;

public class Main {

    public static void main(String[] args) {
        final Scanner in = new Scanner(new BufferedReader(new InputStreamReader(System.in)));
        int r = in.nextInt();
        System.out.println(2.0 * Math.PI * r);
    }
}
