using System;

public class Program

{

    public static void Main()

    {

        Console.WriteLine(GetIndex())

    }

    public static int GetIndex (string str, char value, int startIndex, int count)

    {

        if (str == null)

        { 

             throw new ArgumentNullException(nameof(str), "str cannot be null");

        }

​

        if (string.IsNullOrEmpty(str))

        {

            return -1;

        }

        int endIndex = count + startIndex;

        if (endIndex >= str.Length)

        {

             if (startIndex < str.Length && str[startIndex] == value)

             {

                 return startIndex;

             }

            else 

            {

                return -1;

            }

        }

        if (str[startIndex] == value)

        {

            return startIndex;

        }

        if (startIndex <= endIndex)

        {

            return GetIndex(str, value, startIndex + 1, count - 1);

        }

        return -1;

    }

}