He defined the dissimilarity of two universities X and Y as |x1 − y1| + |x2 − y2| + |x3 − y3| + |x4 − y4| + |x5 − y5|, where an xi (yi) is the number of classes in the requirement i of university X (Y) multiplied by 
an appropriate factor that measures hardness of the corresponding requirement at the corresponding university.(最后这句不懂)