import java.util.Scanner;
public class Main{
	public static void main(String[] args){
      Scanner v = new Scanner(System.in);
      int x = v.nextInt();
      System.out.print(obseg(x));
    }
  public static double obseg(int x){
	return 2*3.1415*x;
  }
}

/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