private static void checkValidSubstraction(String s1, String s2) throws InvalidNumber {
        //int indexPrevChar = index(convertFromString(s2));
        //int indexCurChar = index(convertFromString(s1));

        //if ((indexPrevChar % 2) == 0) {
        //    if ((indexPrevChar - indexCurChar) > 2) {
        //        throw new InvalidNumber();
        //    }
        //} else {
        //    if ((indexPrevChar - indexCurChar) > 1) {
        //        throw new InvalidNumber();
        //    }
        
        //prevChar=s2 | currentChar=s1
        if(s1.equals("I") && (!s2.equals("V")&&!s2.equals("X")) ){
            throw new InvalidNumber();
        }else if(s1.equals("X") && (!s2.equals("L")&&!s2.equals("C")) ){
            throw new InvalidNumber();
        }
        else if(s1.equals("C") && (!s2.equals("D")&&!s2.equals("M")) ){
            throw new InvalidNumber();
        }
    }