Formula to find the Angle between two slopes I have given two slopes $m_1 = \frac{1}{2}$ and $m_2 = 1$

While finding the angle I made use of the formula $\tan(\theta) = \frac{m_1-m_2}{1+m_1m_2}$

answer is : $\theta = \arctan(\frac{-1}{3})$

But in book the answer is $\theta = \arctan(\frac{1}{3})$.

what would be the right answer?
$$
\frac{1-\frac12}{1+1\cdot\frac 12} = \frac 1 3, \qquad \frac{\frac12-1}{1+\frac12\cdot1} =-\frac13.
$$
Either of these is the angle between those two lines.  Where two lines intersect they form angles with measures adding up to $180^\circ$.  If $\alpha+\beta=180^\circ$ then $\tan\alpha = - \tan\beta$.
The angle is acute. Thus, the answer is the book's answer.