Two different numbers are randomly selected from the 30 natural numbers from 1 to 30 so that their sum is an even number. How many different selection methods are there?
在1至30这30个自然数中任意挑选出两个不同的数，使得它们的和是偶数，一共有多少种不同的挑选方法？
<gadget id="calculator">15 * 14</gadget>
<output>210</output>

<gadget id="calculator">2 * 1</gadget>
<output>2</output>

<gadget id="calculator">210 / 2</gadget>
<output>105</output>

<gadget id="calculator">105 + 105</gadget>
<output>210</output>

<result>210</result>