import java.util.Arrays;
import java.util.Comparator;
import java.util.Scanner;


public class Main {
	

	
	
	public static void main(String[] args) {

		
		Scanner s = new Scanner(System.in);
		
		int n = s.nextInt();
		
		System.out.println(2*Math.PI*n);
		
	}
} import java.util.Arrays; Arrays java.util import java.util.Comparator; Comparator java.util import java.util.Scanner; Scanner java.util public class Main {
	

	
	
	public static void main(String[] args) {

		
		Scanner s = new Scanner(System.in);
		
		int n = s.nextInt();
		
		System.out.println(2*Math.PI*n);
		
	}
} Main public static void main(String[] args) {

		
		Scanner s = new Scanner(System.in);
		
		int n = s.nextInt();
		
		System.out.println(2*Math.PI*n);
		
	} main {

		
		Scanner s = new Scanner(System.in);
		
		int n = s.nextInt();
		
		System.out.println(2*Math.PI*n);
		
	} Scanner s = new Scanner(System.in); s new Scanner(System.in) int n = s.nextInt(); n s.nextInt() s.nextInt s System.out.println(2*Math.PI*n) System.out.println System.out System System.out 2*Math.PI*n 2*Math.PI*n 2 Math.PI Math Math.PI n String[] args args public class Main {
	

	
	
	public static void main(String[] args) {

		
		Scanner s = new Scanner(System.in);
		
		int n = s.nextInt();
		
		System.out.println(2*Math.PI*n);
		
	}
} public class Main {
	

	
	
	public static void main(String[] args) {

		
		Scanner s = new Scanner(System.in);
		
		int n = s.nextInt();
		
		System.out.println(2*Math.PI*n);
		
	}
} Main
import java.util.Arrays;
import java.util.Comparator;
import java.util.Scanner;


public class Main {
	

	
	
	public static void main(String[] args) {

		
		Scanner s = new Scanner(System.in);
		
		int n = s.nextInt();
		
		System.out.println(2*Math.PI*n);
		
	}
}