import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;

public class Main
{
	public static void main(String[] Arguments) throws IOException 
	{
		BufferedReader BR = new BufferedReader(new InputStreamReader(System.in));
		int R = Integer.parseInt(BR.readLine());
		System.out.println((double)2 * (double)R * (double)3.1415);
	}
}

/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