find the index of the substring "fir" - Seeni, C#

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//Rextester.Program.Main is the entry point for your code. Don't change it.
//Microsoft (R) Visual C# Compiler version 2.9.0.63208 (958f2354)

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text.RegularExpressions;

namespace Rextester
{
    public class Program
    {
           private static int IndexOf(string str, string substr)
    {
        bool match;

for (int i = 0; i < str.Length - substr.Length + 1; ++i)
        {
            match = true;
            for (int j = 0; j < substr.Length; ++j)
            {
                if (str[i + j] != substr[j])
                {
                    match = false;
                    break;
                }
            }
            if (match) return i;
        }

return -1;
    }
        public static void Main(string[] args)
        {
            //How to find the first index of a substring in a given string?
            //use question as given string and find the index of the substring "fir".
            //Builin array functions and LINQ are not allowed.
            
            Console.WriteLine("Start here...");
             string str = "How to find the first index of a substring in a given string?";
        string substr = "fir";
        int index = IndexOf(str, substr);
        Console.WriteLine(index);
        Console.ReadLine();
                    }
    }
}

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Compilation time: 6,06 sec, absolute running time: 10 sec, cpu time: 0,09 sec, average memory usage: 25 Mb, average nr of threads: 5, absolute service time: 16,08 sec