Strings - Letters Change Numbers

using System;
using System.Collections.Generic;
using System.Globalization;
using System.Linq;
using System.Numerics;

namespace C_
{


    class Program
    {
        static void Main(string[] args)
        {
            var input = Console.ReadLine().Split(new char[] { ' ','\t'},StringSplitOptions.RemoveEmptyEntries).ToList();

            decimal result = 0;

            for (int i = 0; i < input.Count; i++)
            {
                string currentWord = input[i];

                char firstChar = currentWord[0];
                char lastChar = currentWord[currentWord.Length - 1];
                var number = currentWord.Substring(1,currentWord.Length-2).ToString();

                if (isUpper(firstChar))
                {
                    result += decimal.Parse(number) / charPosition(firstChar);
                }
                else
                {
                    result += decimal.Parse(number) * charPosition(firstChar);
                }

                if (isUpper(lastChar))
                {
                    result -= charPosition(lastChar);
                }
                else
                {
                    result += charPosition(lastChar);
                }
            }

            Console.WriteLine($"{result:F2}");

        }

        static bool isUpper (char currentChar)
            {
            int value = (int)currentChar;

            if (value >= 65 && value <=90)
            {
                return true;
            }

            return false;
            }

        static int charPosition (char currentChar)
        {
            char[] alpha = "%ABCDEFGHIJKLMNOPQRSTUVWXYZ".ToCharArray();
            int position = 0;
            for (int i = 0; i < alpha.Length; i++)
            {
                if (currentChar.ToString().ToUpper() == alpha[i].ToString())
                {
                    position = i;
                    break;
                }
            }

            return position;

        }
    }
}