import java.util.*;

public class Main {
    public static void main(String args[]) {
        Scanner scanner = new Scanner(System.in);
        double n = scanner.nextLong();
        double res = n * 2 * Math.PI;
        System.out.println(res);
    }
}
/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