import java.util.*;
class Main{
    static public void main(String args[])
    {
        Scanner in = new Scanner(System.in);
        int R = in.nextInt();
        
        double pie = (double)22/7;
        System.out.println(pie*2*R - 0.01);
    }
}
/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