Index was out of range - не понемаю как такое возмозно - C#

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.IO;
 
namespace lab2_2
{
    class Program
    {
        static void Main(string[] args)
        {
            try
            {
                string f1;
                string f2;
                Console.WriteLine("enter file location for modification:");
                f1 = Console.ReadLine();
                string[] str = File.ReadAllLines(f1);
 
                Console.WriteLine("enter name and path output file");
                f2 = Console.ReadLine();
 
                int b;
                int len = str.Length;
                Console.WriteLine(len);
                using (StreamWriter sw = File.CreateText(f2))
                {
                    sw.AutoFlush = true;
 
                     for (int i = 2; i <len-1; i++)
                      {
 
                          b = str[i].IndexOf(";");
                         str[i].Insert(b,"\n");          // здеся не канает
                         sw.WriteLine(str[i]);
                      }
 
                }
 
                Console.WriteLine("all coments erase!");
                Console.ReadLine();
            }
            catch (FileNotFoundException e)
            {
                Console.WriteLine("File not found! " + e.Message);
                Console.ReadKey();
 
            }
 
        }
    }
}