import java.util.*;

public class Main {// Main
	public static void main(String[] args) throws Exception {
		Scanner sc = new Scanner(System.in);
    double n = sc.nextDouble();
    n = Math.PI*n*2;
    
    System.out.println(n);
	}

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

	
} Main public static void main(String[] args) throws Exception {
		Scanner sc = new Scanner(System.in);
    double n = sc.nextDouble();
    n = Math.PI*n*2;
    
    System.out.println(n);
	} main {
		Scanner sc = new Scanner(System.in);
    double n = sc.nextDouble();
    n = Math.PI*n*2;
    
    System.out.println(n);
	} Scanner sc = new Scanner(System.in); sc new Scanner(System.in) double n = sc.nextDouble(); n sc.nextDouble() sc.nextDouble sc n = Math.PI*n*2 n Math.PI*n*2 Math.PI*n*2 Math.PI Math Math.PI n 2 System.out.println(n) System.out.println System.out System System.out n String[] args args public class Main {// Main
	public static void main(String[] args) throws Exception {
		Scanner sc = new Scanner(System.in);
    double n = sc.nextDouble();
    n = Math.PI*n*2;
    
    System.out.println(n);
	}

	
} public class Main {// Main
	public static void main(String[] args) throws Exception {
		Scanner sc = new Scanner(System.in);
    double n = sc.nextDouble();
    n = Math.PI*n*2;
    
    System.out.println(n);
	}

	
} Main
import java.util.*;

public class Main {// Main
	public static void main(String[] args) throws Exception {
		Scanner sc = new Scanner(System.in);
    double n = sc.nextDouble();
    n = Math.PI*n*2;
    
    System.out.println(n);
	}

	
}