$2 . \operatorname{tg} \frac{\pi}{8}-\operatorname{ctg} \frac{\pi}{8}$ 的值是 $\qquad$ -

The value of $2 . \operatorname{tg} \frac{\pi}{8}-\operatorname{ctg} \frac{\pi}{8}$ is $\qquad$ -
2. $\operatorname{tg} \frac{\pi}{8}-\operatorname{ctg} \frac{\pi}{8}=-2$.

Translate the above text into English, keeping the original text's line breaks and format, and output the translation result directly.

2. $\operatorname{tan} \frac{\pi}{8}-\operatorname{cot} \frac{\pi}{8}=-2$.