

import java.io.BufferedReader;
import java.io.InputStreamReader;


public class Main {

    public static void main(String[] args) {
        
        try {
            InputStreamReader is = new InputStreamReader(System.in);
            BufferedReader br = new BufferedReader(is);
            int i = Integer.parseInt(br.readLine());
            double c =Math.PI*2*i;
            System.out.println(c);
        }catch (Exception e){
            e.printStackTrace();
        }
        
    }
}
    

/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