def add_even_sub_odd(operator, operand):
    """Add even numbers, subtract odd ones. See http://1w6.org/w6 """
    try:
        for i, x in enumerate(operand):
            if x % 2:
                operand[i] = -x
        return operand
    except TypeError:
        if operand % 2:
            return -operand
        return operand
