import java.util.Scanner;

public class Main {
	static int R;
	static double L;
	public static void main(String args[]){
		Scanner scanner =new Scanner(System.in);
		R=scanner.nextInt();
		L=R*2*Math.PI;
		System.out.println(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