In a Pareto chart representing frequencies of error categories, the categories A, B, and C are sorted on the x-axis with x-values $1, 2,$ and $3$. The cumulative percentage line goes from $(0, 0\\%)$ to $(1, 50\\%)$ for A, to $(2, 80\\%)$ for B, and to $(3, 100\\%)$ for C. What is the mathematical slope of the line segment from category 1 (A) to category 2 (B)?