import java.util.*;
import java.io.*;

public class A {
    public static void main(String[] args) throws IOException{
        BufferedReader f = new BufferedReader(new InputStreamReader(System.in));
        PrintWriter out = new PrintWriter(new OutputStreamWriter(System.out));
        out.println(2.0*Math.PI*Double.parseDouble(f.readLine()));

        out.close();
    }
}
