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

class Main {
  public static void main(String[] args) throws IOException {
    BufferedReader reader = new BufferedReader(new InputStreamReader(System.in));
    int R = Integer.parseInt(reader.readLine());
    System.out.println(2*R*Math.acos(-1));
  }
} import java.io.BufferedReader; BufferedReader java.io import java.io.IOException; IOException java.io import java.io.InputStreamReader; InputStreamReader java.io class Main {
  public static void main(String[] args) throws IOException {
    BufferedReader reader = new BufferedReader(new InputStreamReader(System.in));
    int R = Integer.parseInt(reader.readLine());
    System.out.println(2*R*Math.acos(-1));
  }
} Main public static void main(String[] args) throws IOException {
    BufferedReader reader = new BufferedReader(new InputStreamReader(System.in));
    int R = Integer.parseInt(reader.readLine());
    System.out.println(2*R*Math.acos(-1));
  } main {
    BufferedReader reader = new BufferedReader(new InputStreamReader(System.in));
    int R = Integer.parseInt(reader.readLine());
    System.out.println(2*R*Math.acos(-1));
  } BufferedReader reader = new BufferedReader(new InputStreamReader(System.in)); reader new BufferedReader(new InputStreamReader(System.in)) int R = Integer.parseInt(reader.readLine()); R Integer.parseInt(reader.readLine()) Integer.parseInt Integer reader.readLine() reader.readLine reader System.out.println(2*R*Math.acos(-1)) System.out.println System.out System System.out 2*R*Math.acos(-1) 2*R*Math.acos(-1) 2 R Math.acos(-1) Math.acos Math -1 1 String[] args args
import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;

class Main {
  public static void main(String[] args) throws IOException {
    BufferedReader reader = new BufferedReader(new InputStreamReader(System.in));
    int R = Integer.parseInt(reader.readLine());
    System.out.println(2*R*Math.acos(-1));
  }
}