A school has several classes of students, each taking a final exam. You are provided a 2D integer array, classes, where classes[i] = [passi, totali]. Here, passi represents the number of students in the $i^{th}$ class who are expected to pass the exam, and totali represents the total number of students in that class.

Additionally, you are given an integer, extraStudents, which denotes the number of brilliant extra students guaranteed to pass their exams. These students can be assigned to any class, and your goal is to distribute them to maximize the average pass ratio across all classes.

The pass ratio for a class is defined as the ratio of the number of students passing to the total number of students in the class: