Nikulden's_Charity [C#]

using System;

namespace Nikulden_s_Charity
{
    public class Program
    {
        public static void Main(string[] args)
        {
            string message = Console.ReadLine();
            int sumOfSubString = 0;

            string input = string.Empty;

            while (input != "Finish")
            {
                input = Console.ReadLine();

                string[] token = input.Split(" ", StringSplitOptions.RemoveEmptyEntries);
                string command = token[0];

                if (command == "Replace")
                {
                    string currentChar = token[1];
                    string newChar = token[2];

                    if (message.Contains(currentChar))
                    {
                        message = message.Replace(currentChar, newChar);
                    }

                    Console.WriteLine(message);
                }

                else if (command == "Cut")
                {
                    int startIdx = int.Parse(token[1]);
                    int endIdx = int.Parse(token[2]);
                    int length = (endIdx - startIdx) + 1;

                    if (startIdx >= 0 && endIdx < message.Length)
                    {
                        message = message.Remove(startIdx, length);

                        Console.WriteLine(message);
                    }
                    else
                    {
                        Console.WriteLine("Invalid indexes!");
                    }
                }

                else if (command == "Make")
                {
                    if (token[1] == "Upper")
                    {
                        message = message.ToUpper();
                    }
                    else
                    {
                        message = message.ToLower();
                    }

                    Console.WriteLine(message);
                }

                else if (command == "Check")
                {
                    string str = token[1];

                    if (message.Contains(str))
                    {
                        Console.WriteLine($"Message contains {str}");
                    }

                    else
                    {
                        Console.WriteLine($"Message doesn't contain {str}");
                    }
                }

                else if (command == "Sum")
                {
                    int startIdx = int.Parse(token[1]);
                    int endIdx = int.Parse(token[2]);
                    int length = (endIdx - startIdx) + 1;

                    if (startIdx >= 0 && endIdx < message.Length)
                    {
                        string substr = message.Substring(startIdx, length);

                        for (int i = 0; i < substr.Length; i++)
                        {
                            sumOfSubString += substr[i];
                        }
                        Console.WriteLine(sumOfSubString);
                    }

                    else
                    {
                        Console.WriteLine("Invalid indexes!");
                    }
                }
            }
        }
    }
}