import java.util.*;
public class Main {

    public static void main(String[] args) {
        Scanner  yomi = new Scanner(System.in);
        double n = yomi.nextDouble();
        System.out.println(n*2*3.1415);

    }
}
/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