Determine the maximum number of letters within "s" that are between two "programmer strings"

StringProgrammer.cs

private static char[] CheckerHelper() => "programmer".ToCharArray();

//Time: O(n) where n is the length  of s
//Memory: O(1) using a fixed length char[] checkerHelper
static int StringProgrammer(string s)
{
    var checkerHelper = CheckerHelper();
    var lettersFound = 0;

    for(var i = 0; i < s.Length; i++)
    {
        var index = Array.IndexOf(checkerHelper, s[i]);

        if (index > -1) //current letter exists in 'programmer' checkerHelper
        {
            checkerHelper[index] = '0'; //mark letter as found in checkerHelper
            lettersFound++;


            if (lettersFound == 10) //found first programmer
            {
                //when find the first programmer
                //reset checkerHelper to its initial state
                //reverse the ramaining chars of s
                //reset i to initial its state.

                //result will be the amount of chars ramaining after
                //find the second programmer

                checkerHelper = CheckerHelper();
                var sTemp = s.Substring(i + 1).ToCharArray();
                Array.Reverse(sTemp);
                s = new string(sTemp);
                i = -1;
            }
            else if (lettersFound == 20) //found second programmer
                return s.Length - i - 1;
        }
    }

    return -1;
}

static void Main(string[] args)
{
    //StringProgrammer("xprogxrmaxemrppprmmograeiruu");
    //2
    
    //StringProgrammer("progxrammerrxproxgrammer");
    //2
    
    //Console.WriteLine(StringProgrammer("programmerlprogrammer"));
    //1
    
    //StringProgrammer("programmerprogrammer");
    //0
}