import java.util.*;
import java.util.Scanner;
public class Main {
	public static void main(String[] args) {
		Scanner scan = new Scanner(System.in);
		
		double r = scan.nextDouble();
		double x = Math.PI;
		System.out.println(r*2*x);
		
	}	
	
	
	//最小公約数
	/*
	static int gcd (int a, int b) {
		int temp;
		while((temp = a%b)!=0) {
			a = b;
			b = temp;
		}
		return b;
	}
	*/
	
} import java.util.*; util.* java import java.util.Scanner; Scanner java.util public class Main {
	public static void main(String[] args) {
		Scanner scan = new Scanner(System.in);
		
		double r = scan.nextDouble();
		double x = Math.PI;
		System.out.println(r*2*x);
		
	}	
	
	
	//最小公約数
	/*
	static int gcd (int a, int b) {
		int temp;
		while((temp = a%b)!=0) {
			a = b;
			b = temp;
		}
		return b;
	}
	*/
	
} Main public static void main(String[] args) {
		Scanner scan = new Scanner(System.in);
		
		double r = scan.nextDouble();
		double x = Math.PI;
		System.out.println(r*2*x);
		
	} main {
		Scanner scan = new Scanner(System.in);
		
		double r = scan.nextDouble();
		double x = Math.PI;
		System.out.println(r*2*x);
		
	} Scanner scan = new Scanner(System.in); scan new Scanner(System.in) double r = scan.nextDouble(); r scan.nextDouble() scan.nextDouble scan double x = Math.PI; x Math.PI Math Math.PI System.out.println(r*2*x) System.out.println System.out System System.out r*2*x r*2*x r 2 x String[] args args public class Main {
	public static void main(String[] args) {
		Scanner scan = new Scanner(System.in);
		
		double r = scan.nextDouble();
		double x = Math.PI;
		System.out.println(r*2*x);
		
	}	
	
	
	//最小公約数
	/*
	static int gcd (int a, int b) {
		int temp;
		while((temp = a%b)!=0) {
			a = b;
			b = temp;
		}
		return b;
	}
	*/
	
} public class Main {
	public static void main(String[] args) {
		Scanner scan = new Scanner(System.in);
		
		double r = scan.nextDouble();
		double x = Math.PI;
		System.out.println(r*2*x);
		
	}	
	
	
	//最小公約数
	/*
	static int gcd (int a, int b) {
		int temp;
		while((temp = a%b)!=0) {
			a = b;
			b = temp;
		}
		return b;
	}
	*/
	
} Main
import java.util.*;
import java.util.Scanner;
public class Main {
	public static void main(String[] args) {
		Scanner scan = new Scanner(System.in);
		
		double r = scan.nextDouble();
		double x = Math.PI;
		System.out.println(r*2*x);
		
	}	
	
	
	//最小公約数
	/*
	static int gcd (int a, int b) {
		int temp;
		while((temp = a%b)!=0) {
			a = b;
			b = temp;
		}
		return b;
	}
	*/
	
}