import java.io.IOException;
import java.io.InputStreamReader;
import java.io.BufferedReader;
class TestClass {
    public static void main(String args[] ) throws Exception {
//        int testCases = Integer.parseInt(br.readLine());
//        while(testCases-- >0){
//
//        }
        double in = GetInput.getInt();
        System.out.println(Math.PI*2*in);
    }
}

class GetInput {
    static BufferedReader bufferedReader = new BufferedReader(new InputStreamReader(System.in));

    static char[] getCharArray() {
        char[] charArray;
        try {
            StringBuilder string = getInputString();
            charArray = new char[string.length()];
            for (int i = 0; i < string.length(); i++) {
                charArray[i] = string.charAt(i);
            }
            return charArray;
        } catch (Exception e) {
            e.printStackTrace();
        }
        charArray = new char[0];
        return charArray;
    }

    static int[] getArrayInt() {
        String[] string;
        int[] array;
        try {
            string = bufferedReader.readLine().split("\\s+");
            array = new int[string.length];
            for (int i = 0; i < string.length; i++) {
                array[i] = Integer.parseInt(string[i]);
            }
            return array;
        } catch (IOException e) {
            e.printStackTrace();
        }

        int[] arra = new int[2];
        return arra;
    }

    static int getInt() {
        try {
            String string = bufferedReader.readLine();
            return Integer.parseInt(string);
        } catch (IOException e) {
            e.printStackTrace();
        }

        return 0;
    }

    static StringBuilder getInputString() {
        try {
            StringBuilder string = new StringBuilder();
            string.append(bufferedReader.readLine());
            return string;
        } catch (IOException e) {
            e.printStackTrace();
        }
        return new StringBuilder();
    }

    static long getLongInput() {
        try {
            String string = bufferedReader.readLine();
            return Long.parseLong(string);
        } catch (IOException e) {
            e.printStackTrace();
        }

        return 0;
    }

    static long[] getLongArrayInput() {
        String[] string;
        long[] array;
        try {
            string = bufferedReader.readLine().split("\\s+");
            array = new long[string.length];
            for (int i = 0; i < string.length; i++) {
                array[i] = Long.parseLong(string[i]);
            }
            return array;
        } catch (IOException e) {
            e.printStackTrace();
        }

        long[] arra = new long[2];
        return arra;
    }
}

