Change_to_Uppercase

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace _6.Change_to_Uppercase
{
    class Program
    {
        static void Main(string[] args)
        {
            string text = Console.ReadLine().Trim();
            int upcaseStart = text.IndexOf("<upcase>");
            int upcaseFinish = text.IndexOf("</upcase>");
            string replacement = text.Substring(upcaseStart + 8, upcaseFinish - upcaseStart - 8);
            replacement = replacement.ToUpper();
            string toDelete = text.Substring(upcaseStart, upcaseFinish + 8 + 1 - upcaseStart);

            while (text.Contains("<upcase>"))
            {
                text = text.Replace(toDelete, replacement);
                upcaseStart = text.IndexOf("<upcase>");
                upcaseFinish = text.IndexOf("</upcase>");
                if (upcaseStart != -1 && upcaseFinish != -1)
                {
                    replacement = text.Substring(upcaseStart + 8, upcaseFinish - upcaseStart - 8);
                    replacement = replacement.ToUpper();
                    toDelete = text.Substring(upcaseStart, upcaseFinish + 8 + 1 - upcaseStart);
                }

            }

            Console.WriteLine(text);

        }
    }
}