using System;

using System.Globalization;

using System.Numerics;

​

public class Program

{

    //non recursive implementation to cope with process and memory optimization when calculate big numbers

    public static BigInteger specialMath(int n)

    {

        if (n == 0 || n == 1)

            return n;

        BigInteger prev = 0;

        BigInteger prevPrev = 1;

        BigInteger fiboLike = 0;

        for (var i = 2; i <= n; i++)

        {

            fiboLike = i + prev + prevPrev;

            //if you want to see intermediate values, uncomment code below

            //Console.WriteLine("i = " + i + " sm = " + fiboLike);

            prev = prevPrev;

            prevPrev = fiboLike;

        }

​

        return fiboLike;

    }

​

    public static void Main()

    {

        Console.WriteLine("specialMath {0} -> {1}", 7, specialMath(7));

        Console.WriteLine("specialMath {0} -> {1}", 17, specialMath(17));

        Console.WriteLine("specialMath {0} -> {1}", 90, specialMath(90));

    }

}