31. Given the difference of squares formula: $a^{2}-b^{2}=(a+b) \times(a-b)$. Calculate:
$$
99^{2}+97^{2}-95^{2}-93^{2}+91^{2}+89^{2}-87^{2}-85^{2}+\cdots+11^{2}+9^{2}-7^{2}-5^{2}=
$$
$\qquad$
Answer: 9984