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


public class hef {
	

	
	
	public static void main(String[] args) {

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