
import java.util.*;
import java.io.*;

public class Main {
	// https://atcoder.jp/contests/abc163/tasks/abc163_a
	
	public static void main(String[] args) throws IOException, FileNotFoundException {
		BufferedReader in = new BufferedReader(new InputStreamReader(System.in));
		//BufferedReader in = new BufferedReader(new FileReader("CirclePond"));

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

	}

} import java.util.*; util.* java import java.io.*; io.* java public class Main {
	// https://atcoder.jp/contests/abc163/tasks/abc163_a
	
	public static void main(String[] args) throws IOException, FileNotFoundException {
		BufferedReader in = new BufferedReader(new InputStreamReader(System.in));
		//BufferedReader in = new BufferedReader(new FileReader("CirclePond"));

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

	}

} Main public static void main(String[] args) throws IOException, FileNotFoundException {
		BufferedReader in = new BufferedReader(new InputStreamReader(System.in));
		//BufferedReader in = new BufferedReader(new FileReader("CirclePond"));

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

	} main {
		BufferedReader in = new BufferedReader(new InputStreamReader(System.in));
		//BufferedReader in = new BufferedReader(new FileReader("CirclePond"));

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

	} BufferedReader in = new BufferedReader(new InputStreamReader(System.in)); in new BufferedReader(new InputStreamReader(System.in)) int r = Integer.parseInt(in.readLine()); r Integer.parseInt(in.readLine()) Integer.parseInt Integer in.readLine() in.readLine in System.out.println(2 * r * Math.PI) System.out.println System.out System System.out 2 * r * Math.PI 2 * r * Math.PI 2 r Math.PI Math Math.PI String[] args args public class Main {
	// https://atcoder.jp/contests/abc163/tasks/abc163_a
	
	public static void main(String[] args) throws IOException, FileNotFoundException {
		BufferedReader in = new BufferedReader(new InputStreamReader(System.in));
		//BufferedReader in = new BufferedReader(new FileReader("CirclePond"));

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

	}

} public class Main {
	// https://atcoder.jp/contests/abc163/tasks/abc163_a
	
	public static void main(String[] args) throws IOException, FileNotFoundException {
		BufferedReader in = new BufferedReader(new InputStreamReader(System.in));
		//BufferedReader in = new BufferedReader(new FileReader("CirclePond"));

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

	}

} Main

import java.util.*;
import java.io.*;

public class Main {
	// https://atcoder.jp/contests/abc163/tasks/abc163_a
	
	public static void main(String[] args) throws IOException, FileNotFoundException {
		BufferedReader in = new BufferedReader(new InputStreamReader(System.in));
		//BufferedReader in = new BufferedReader(new FileReader("CirclePond"));

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

	}

}
