From the 10 numbers from 1 to 10, how many ways are there to choose two different numbers so that their sum is an even number?
从1～10这10个数中，选取两个不同的数，使其和为偶数的选法有多少种．
<gadget id="calculator">5 * 4</gadget>
<output>20</output>

<gadget id="calculator">20 + 20</gadget>
<output>40</output>

<result>40</result>