Eighteen people are standing in a (socially-distanced) line to enter a grocery store. Five people are wearing a black mask, 6 are wearing a gray mask, and 7 are wearing a white mask. Suppose that these 18 people got on line in a random order. The expected number of pairs of adjacent people wearing different-colored masks can be given by \frac{a}{b}, where \operatorname{gcd}(a, b)=1. Compute a+b.