import java.util.*;
import java.util.Collections;

class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int R = sc.nextInt();

        //double PI = 3.1415926535897932;
        double PI = 3.14159265;
        double v = 2 *R* PI;

        System.out.println(v);

    }
}
/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