import java.util.*;

class Main{
	static public void main(String[] args){
		Scanner sc=new Scanner(System.in);
		
		double r=sc.nextDouble();
		
		System.out.println(r*2*3.14159265359);
	
	}
} import java.util.*; util.* java class Main{
	static public void main(String[] args){
		Scanner sc=new Scanner(System.in);
		
		double r=sc.nextDouble();
		
		System.out.println(r*2*3.14159265359);
	
	}
} Main static public void main(String[] args){
		Scanner sc=new Scanner(System.in);
		
		double r=sc.nextDouble();
		
		System.out.println(r*2*3.14159265359);
	
	} main {
		Scanner sc=new Scanner(System.in);
		
		double r=sc.nextDouble();
		
		System.out.println(r*2*3.14159265359);
	
	} 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*3.14159265359) System.out.println System.out System System.out r*2*3.14159265359 r*2*3.14159265359 r 2 3.14159265359 String[] args args
import java.util.*;

class Main{
	static public void main(String[] args){
		Scanner sc=new Scanner(System.in);
		
		double r=sc.nextDouble();
		
		System.out.println(r*2*3.14159265359);
	
	}
}