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

public class Main {
    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();
    }
} import java.util.*; util.* java import java.io.*; io.* java public class Main {
    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();
    }
} Main 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();
    } main {
        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();
    } BufferedReader f = new BufferedReader(new InputStreamReader(System.in)); f new BufferedReader(new InputStreamReader(System.in)) PrintWriter out = new PrintWriter(new OutputStreamWriter(System.out)); out new PrintWriter(new OutputStreamWriter(System.out)) out.println(2.0*Math.PI*Double.parseDouble(f.readLine())) out.println out 2.0*Math.PI*Double.parseDouble(f.readLine()) 2.0*Math.PI*Double.parseDouble(f.readLine()) 2.0 Math.PI Math Math.PI Double.parseDouble(f.readLine()) Double.parseDouble Double f.readLine() f.readLine f out.close() out.close out String[] args args public class Main {
    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();
    }
} public class Main {
    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();
    }
} Main
import java.util.*;
import java.io.*;

public class Main {
    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();
    }
}
