import java.util.*;
import java.math.BigDecimal;

public class Main{
    public static void main(String[] args){
        BigDecimal A, B;
        Scanner sc = new Scanner(System.in);
        A =sc.nextBigDecimal();
        B = new BigDecimal(2).multiply(A).multiply(new BigDecimal(3.14159265359));
        System.out.println(B);
    }
}


/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