using System;

public class Program

{

    public static void Main()

    {

        Console.WriteLine("Nhap N:");

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

        double kq=0;

        int i=1;

        while(i<=N)

            {kq=Math.Sqrt(2+kq);

            i++;

            }

        Console.WriteLine(kq);

    }

}