import java.io.*;

class Main{
	public static void main(String[] args){
      
      BufferedReader br = new BufferedReader(new InputStreamReader(System.in) );
  	try{
  			String str = br.readLine();
  			int num = Integer.parseInt(str);
  
  			double p = 3.14;
  
  			System.out.println(2.0*p*num);
       }catch(Exception e){
 	   }
   }
}
/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