A decimal number can be converted to its Hexspeak representation by first converting it to an uppercase hexadecimal string, then replacing all occurrences of the digit 0 with the letter O, and the digit 1 with the letter I. Such a representation is valid if and only if it consists only of the letters in the set {"A", "B", "C", "D", "E", "F", "I", "O"}. Given a string num representing a decimal integer N, return the Hexspeak representation of N if it is valid, otherwise return "ERROR". Example 1: Input: num = "257" Output: "IOI" Explanation: 257 is 101 in hexadecimal. Example 2: Input: num = "3" Output: "ERROR" Constraints: 1 <= N <= 10^12 There are no leading zeros in the given string. All answers must be in uppercase letters.