Select several different numbers from the six numbers 1, 2, 3, 4, 5, and 6 so that their sum is an even number. How many different ways are there to choose?
从1、2、3、4、5、6这六个数中选出几个不同的数，使他们的和是偶数，共有多少种不同的选法．
<gadget id="calculator">6 + 9 + 9 + 3</gadget>
<output>27</output>

<result>27</result>