ITF Barcode Algorithm

Question

I have this C# ITF/Interleaved 2 of 5 algorithm (from a VB Azalea Software algorithm):

public static string Interleaved25(string input)
{
    if (input.Length <= 0) return "";

    if (input.Length % 2 != 0)
    {
        input = "0" + input;
    }

    string result = "";

    //Takes pairs of numbers and convert them to chars
    for (int i = 0; i <= input.Length - 1; i += 2)
    {
        int pair = Int32.Parse(input.Substring(i, 2));

        if (pair < 90) 
            pair = pair + 33;
        else if (pair == 90)
            pair = pair + 182;
       else if (pair == 91)
            pair = pair + 183;
       else if (pair > 91)
            pair = pair + 104;

       result = result + Convert.ToChar(pair);
   }

   //Leading and trailing chars.
   return (char)171 + result + (char)172;
}

The problem is that somehow chars with value > 89 all have empty boxes as result (using the ITF font).

Answer 1

While typing this question, I got the answer all by myself.

Here is the new code:

if (pair < 90)
    pair = pair + 33;
else
    {
        pair = pair + 71;
    }

Basically, all chars between 90 and 99 needs a + 71.