Take two different numbers from the natural numbers 1, 2, 3...500, the sum of which is a multiple of 10, how many different ways are there in total?
从自然数1、2、3…500中取出两个不同的数，其和是10的倍数，一共有多少种不同的取法？
<gadget id="calculator">50 * 50</gadget>
<output>2_500</output>

<gadget id="calculator">2_500 * 4</gadget>
<output>10_000</output>

<gadget id="calculator">50 * 49</gadget>
<output>2_450</output>

<gadget id="calculator">2_450 * 2</gadget>
<output>4_900</output>

<gadget id="calculator">10_000 + 4_900</gadget>
<output>14_900</output>

<result>14_900</result>