21. A number that reads the same forward and backward is called a palindrome. For example, 121 is a three-digit palindrome, and 5665 is a four-digit palindrome. The sum of two four-digit palindromes is a five-digit palindrome. There are $\qquad$ pairs of such four-digit palindromes. (Note: Two numbers count as 1 pair, and the order of the numbers does not matter, for example, 2 and 3, 3 and 2, count as 1 pair)
$36$