import java.util.Scanner;
public class Main {
   
    public static void main(String[] args) { 
		Scanner sc = new Scanner(System.in);
double R=sc.nextDouble();
        System.out.printf("%.2f",2*3.14159*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