import java.util.Scanner;

class Main{
    public static void main(String[] args){
        Scanner stdIn = new Scanner(System.in);
        int R = stdIn.nextInt();
        double pie = 3.141952;
        double L = 0.0;
        L = 2 * pie * (double)R;
        System.out.print(L);

    }
} 
/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