import java.util.*;

public class Main {
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		Double num = sc.nextDouble();
      	System.out.println(num*3.1415926535897932*2.0);
	}
} import java.util.*; util.* java public class Main {
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		Double num = sc.nextDouble();
      	System.out.println(num*3.1415926535897932*2.0);
	}
} Main public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		Double num = sc.nextDouble();
      	System.out.println(num*3.1415926535897932*2.0);
	} main {
		Scanner sc = new Scanner(System.in);
		Double num = sc.nextDouble();
      	System.out.println(num*3.1415926535897932*2.0);
	} Scanner sc = new Scanner(System.in); sc new Scanner(System.in) Double num = sc.nextDouble(); num sc.nextDouble() sc.nextDouble sc System.out.println(num*3.1415926535897932*2.0) System.out.println System.out System System.out num*3.1415926535897932*2.0 num*3.1415926535897932*2.0 num 3.1415926535897932 2.0 String[] args args public class Main {
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		Double num = sc.nextDouble();
      	System.out.println(num*3.1415926535897932*2.0);
	}
} public class Main {
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		Double num = sc.nextDouble();
      	System.out.println(num*3.1415926535897932*2.0);
	}
} Main
import java.util.*;

public class Main {
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		Double num = sc.nextDouble();
      	System.out.println(num*3.1415926535897932*2.0);
	}
}