import java.util.Scanner;

public class  Main {
	public static void main(String[] args) {
		double r;
		double area;
		Scanner scanner = new Scanner(System.in);
		
		r = scanner.nextDouble();
		
		area = r * 3.14 * 2;
		
		System.out.println(area);
	}
}
/tmp/hf-datasets-cache/medium/datasets/42704881117311-config-parquet-and-info-qiankunmu-Project_CodeNet-896c9d36/hub/datasets--qiankunmu--Project_CodeNet_Python800_and_Java250/snapshots/c899fde44a77ba1cc14973ed3de064e4ff3aadc7/Project_CodeNet_Java250.tar.gz