/// <summary>
/// Calculate Levenshtein distance between two strings
/// </summary>
/// <param name="str1">The string 1.</param>
/// <param name="str2">The string 2.</param>
/// <returns>
/// This function returns the Levenshtein-Distance between the two argument 
/// strings or -1, if one of the argument strings is longer than the limit 
/// of 255 characters.
/// </returns>
public static int Levenshtein(string str1, string str2)