import java.util.Scanner;
import java.math.BigDecimal;

public class Main {
	public static void main(String[] args) {
      	Scanner scan = new Scanner(System.in);
      	BigDecimal pi = new BigDecimal("3.14");
		BigDecimal in = new BigDecimal(scan.nextLine());
    	System.out.println(pi.multiply(in).multiply(new BigDecimal("2")));
    }
}
/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