Let $S$ be the set of points with polar coordinates $(r, \theta),$ where $1 \le r \le 2$ and $0 \le \theta \le \frac{\pi}{2}.$  Compute the area of $S.$
<|im_start|>system
You are a helpful assistant.<|im_end|>
<|im_start|>user
Let $S$ be the set of points with polar coordinates $(r, \theta),$ where $1 \le r \le 2$ and $0 \le \theta \le \frac{\pi}{2}.$  Compute the area of $S.$
Please reason step by step, and put your final answer within \boxed{{}}.<|im_end|>
<|im_start|>assistant

user
\frac{3 \pi}{4}
Let $S$ be the set of points with polar coordinates $(r, \theta),$ where $1 \le r \le 2$ and $0 \le \theta \le \frac{\pi}{2}.$  Compute the area of $S.$
train