using System;

​

public class Program

{

    public static void Main()

    {

        double result = 1;

        Console.WriteLine("Number of iterations:");

        int input = int.Parse(Console.ReadLine());

        for (int i = 1; i < input; i++)

        {

            if (i % 2 == 0)

            {

                result += (1 / (1.0 + (2 * i)));

            }

            else

            {

                result -= (1 / (1.0 + (2 * i)));

            }

        }

​

        Console.WriteLine(result * 4);

    }

}