import java.util.*;
import java.lang.*;
 
public class Main {
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		double r = sc.nextDouble();
		System.out.println(r * 2 * Math.PI);
	}
} import java.util.*; util.* java import java.lang.*; lang.* java public class Main {
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		double r = sc.nextDouble();
		System.out.println(r * 2 * Math.PI);
	}
} Main public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		double r = sc.nextDouble();
		System.out.println(r * 2 * Math.PI);
	} main {
		Scanner sc = new Scanner(System.in);
		double r = sc.nextDouble();
		System.out.println(r * 2 * Math.PI);
	} Scanner sc = new Scanner(System.in); sc new Scanner(System.in) double r = sc.nextDouble(); r sc.nextDouble() sc.nextDouble sc System.out.println(r * 2 * Math.PI) System.out.println System.out System System.out r * 2 * Math.PI r * 2 * Math.PI r 2 Math.PI Math Math.PI String[] args args public class Main {
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		double r = sc.nextDouble();
		System.out.println(r * 2 * Math.PI);
	}
} public class Main {
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		double r = sc.nextDouble();
		System.out.println(r * 2 * Math.PI);
	}
} Main
import java.util.*;
import java.lang.*;
 
public class Main {
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		double r = sc.nextDouble();
		System.out.println(r * 2 * Math.PI);
	}
}