import static java.lang.Math.acos;
import java.util.Scanner;
public class Main {
    public static void main(String[] args) {
        Scanner in=new Scanner(System.in);
        double r=in.nextDouble();
        double pi=acos(-1.0);
        Double are=2*pi*r;
        System.out.printf("%.20f\n",are);
        
    }
} import static java.lang.Math.acos; import static java.lang.Math.acos; import static java.lang.Math.acos; import java.util.Scanner; Scanner java.util public class Main {
    public static void main(String[] args) {
        Scanner in=new Scanner(System.in);
        double r=in.nextDouble();
        double pi=acos(-1.0);
        Double are=2*pi*r;
        System.out.printf("%.20f\n",are);
        
    }
} Main public static void main(String[] args) {
        Scanner in=new Scanner(System.in);
        double r=in.nextDouble();
        double pi=acos(-1.0);
        Double are=2*pi*r;
        System.out.printf("%.20f\n",are);
        
    } main {
        Scanner in=new Scanner(System.in);
        double r=in.nextDouble();
        double pi=acos(-1.0);
        Double are=2*pi*r;
        System.out.printf("%.20f\n",are);
        
    } Scanner in=new Scanner(System.in); in new Scanner(System.in) double r=in.nextDouble(); r in.nextDouble() in.nextDouble in double pi=acos(-1.0); pi acos(-1.0) acos -1.0 1.0 Double are=2*pi*r; are 2*pi*r 2*pi*r 2 pi r System.out.printf("%.20f\n",are) System.out.printf System.out System System.out "%.20f\n" are String[] args args public class Main {
    public static void main(String[] args) {
        Scanner in=new Scanner(System.in);
        double r=in.nextDouble();
        double pi=acos(-1.0);
        Double are=2*pi*r;
        System.out.printf("%.20f\n",are);
        
    }
} public class Main {
    public static void main(String[] args) {
        Scanner in=new Scanner(System.in);
        double r=in.nextDouble();
        double pi=acos(-1.0);
        Double are=2*pi*r;
        System.out.printf("%.20f\n",are);
        
    }
} Main
import static java.lang.Math.acos;
import java.util.Scanner;
public class Main {
    public static void main(String[] args) {
        Scanner in=new Scanner(System.in);
        double r=in.nextDouble();
        double pi=acos(-1.0);
        Double are=2*pi*r;
        System.out.printf("%.20f\n",are);
        
    }
}