The slopes of perpendicular lines are negative reciprocals. Therefore, the slope of line $ b $ is the negative reciprocal of $ -2 $, which is $ \frac12 $. The slopes of parallel lines are the same, therefore the slope of line $ a $ is also $ \frac12 $. Using the point-slope formula, the equation of line $ a $ is $ y-2=\frac12(x-1) $. Changing the form to slope-intercept, we get the equation $ y=\frac{x}{2}+\frac32 $. Therefore, the y-intercept is $ \boxed{\frac32} $.