import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        Long R;
        Scanner sc = new Scanner(System.in);
        R = sc.nextLong();
        double Chu_vi =+R*2*Math.PI;
        //String.format("%.25g%n", Chu_vi);
        System.out.println(Chu_vi);

    }
}

/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