import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;

public class Main {
	public static void main(String[] args) throws IOException {
		try (BufferedReader br = new BufferedReader(
				new InputStreamReader(System.in))) {

			int in = Integer.parseInt(br.readLine());
			System.out.println(in * 2 * Math.PI);
		}
	}
} import java.io.BufferedReader; BufferedReader java.io import java.io.IOException; IOException java.io import java.io.InputStreamReader; InputStreamReader java.io public class Main {
	public static void main(String[] args) throws IOException {
		try (BufferedReader br = new BufferedReader(
				new InputStreamReader(System.in))) {

			int in = Integer.parseInt(br.readLine());
			System.out.println(in * 2 * Math.PI);
		}
	}
} Main public static void main(String[] args) throws IOException {
		try (BufferedReader br = new BufferedReader(
				new InputStreamReader(System.in))) {

			int in = Integer.parseInt(br.readLine());
			System.out.println(in * 2 * Math.PI);
		}
	} main {
		try (BufferedReader br = new BufferedReader(
				new InputStreamReader(System.in))) {

			int in = Integer.parseInt(br.readLine());
			System.out.println(in * 2 * Math.PI);
		}
	} try (BufferedReader br = new BufferedReader(
				new InputStreamReader(System.in))) {

			int in = Integer.parseInt(br.readLine());
			System.out.println(in * 2 * Math.PI);
		} {

			int in = Integer.parseInt(br.readLine());
			System.out.println(in * 2 * Math.PI);
		} BufferedReader br = new BufferedReader(
				new InputStreamReader(System.in)) BufferedReader br = new BufferedReader(
				new InputStreamReader(System.in)) new BufferedReader(
				new InputStreamReader(System.in)) int in = Integer.parseInt(br.readLine()); in Integer.parseInt(br.readLine()) Integer.parseInt Integer br.readLine() br.readLine br System.out.println(in * 2 * Math.PI) System.out.println System.out System System.out in * 2 * Math.PI in * 2 * Math.PI in 2 Math.PI Math Math.PI String[] args args public class Main {
	public static void main(String[] args) throws IOException {
		try (BufferedReader br = new BufferedReader(
				new InputStreamReader(System.in))) {

			int in = Integer.parseInt(br.readLine());
			System.out.println(in * 2 * Math.PI);
		}
	}
} public class Main {
	public static void main(String[] args) throws IOException {
		try (BufferedReader br = new BufferedReader(
				new InputStreamReader(System.in))) {

			int in = Integer.parseInt(br.readLine());
			System.out.println(in * 2 * Math.PI);
		}
	}
} Main
import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;

public class Main {
	public static void main(String[] args) throws IOException {
		try (BufferedReader br = new BufferedReader(
				new InputStreamReader(System.in))) {

			int in = Integer.parseInt(br.readLine());
			System.out.println(in * 2 * Math.PI);
		}
	}
}