import java.util.*;
import java.util.Collections;
import java.util.stream.Collectors;
 
public class Main {
 
  public static void main(String[] args) {

	Scanner s = new Scanner(System.in);
		int num = s.nextInt();
		Double r = (double) num;
	System.out.println(2*3.14*r);
  }
}
/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