check the picture below, we'll use those two points.
[tex]\bf (\stackrel{x_1}{0}~,~\stackrel{y_1}{-1})\qquad
(\stackrel{x_2}{4}~,~\stackrel{y_2}{1})
\\\\\\
slope = m\implies
\cfrac{\stackrel{rise}{ y_2- y_1}}{\stackrel{run}{ x_2- x_1}}\implies \cfrac{1-(-1)}{4-0}\implies \cfrac{1+1}{4-0}\implies \cfrac{2}{4}\implies \cfrac{1}{2}[/tex]
