Hi Elena,

The exercise can only be solve when using: totalSum = Math.Round(totalSum, 2, MidpointRounding.AwayFromZero);

Strangely enough, Midpoint Rounding is never used again in the latter exercises, so I guess it is solely used for some demo-purpose. Otherwise your code seems OK so that should be the only problem.

using System;
using System.Text;

namespace lettersChangeNumbers
{
    class Program
    {
        static void Main(string[] args)
        {
            string[] input = Console.ReadLine().Split(" ", StringSplitOptions.RemoveEmptyEntries);
            double totalSum = 0;


            for (int i = 0; i < input.Length; i++)
            {
                StringBuilder sb = new StringBuilder(input[i]);
                char firstLetter = sb[0];
                char lastLetter = sb[^1];
                string numbers = input[i].Substring(1, input[i].Length - 2);
                double num = int.Parse(numbers);
                if ((int)firstLetter >= 65 && (int)firstLetter <= 90)
                {
                    num = FirstCapitalLetter(firstLetter, num);
                }
                else if ((int)firstLetter >= 97 && (int)firstLetter <= 122)
                {
                    num = FirstLowerLetter(firstLetter, num);
                }
                if ((int)lastLetter >= 65 && (int)lastLetter <= 90)
                {
                    num = SecondCapitalLetter(lastLetter, num);
                }
                else if ((int)lastLetter >= 97 && (int)lastLetter <= 122)
                {
                    num = SecondLowerLetter(lastLetter, num);
                }

                totalSum += num;
                
            }
            totalSum = Math.Round(totalSum, 2, MidpointRounding.AwayFromZero);
            Console.WriteLine($"{totalSum:F2}");
        }

        static double FirstCapitalLetter(char firstLetter, double num)
        {
            int firstL = (int)firstLetter - 64;
            num = num / firstL;
            return num;
        }

        static double FirstLowerLetter(char firstLetter, double num)
        {
            int firstL = (int)firstLetter - 96;
            num = num * firstL;
            return num;
        }

        static double SecondCapitalLetter(char lastLetter, double num)
        {
            int lastL = (int)lastLetter - 64;
            num = num - lastL;
            return num;
        }

        static double SecondLowerLetter(char lastLetter, double num)
        {
            int lastL = (int)lastLetter - 96;
            num = num + lastL;
            return num;
        }
    }
}