using System;

public class Program

{

    public static void Main()

    {

        int n = 23;

        Console.WriteLine($"Ascendant({n}): {Algorithms.Ascendant(23)}");

    }

}

​

public static class Algorithms

{

    public static int? Ascendant(int n)

    {

        if (n < 2)

        {

            return null;

        }

        if (n == 2)

        {

            return 1;

        }

        var root = 2;

        var left = root + 1;

        var right = root + root;

        while (right < n)

        {

            Console.WriteLine($"{root} = [{left}|{right}]");

            root += 1;

            left = right + 1;

            right = right + root;

        }

        return root;

    }

}