The slope-intercept form of the equation of a straight line is y=mx+b, where m is the slope and b is the y-coordinate of the y-intercept.
If the given slope is -1/2, then this equation becomes y = (-1/2)x + b.
You do not say that b=1.
But if we let b=1, then the equation becomes y = (-1/2)x + 1.