You are given a 0-indexed 2D integer array `questions` where `questions[i] = [pointsi, brainpoweri]`.

The array describes the questions of an exam, where you have to process the questions in order (i.e., starting from question `0`) and make a decision whether to solve or skip each question. Solving question `i` will earn you `pointsi` points but you will be unable to solve each of the next `brainpoweri` questions. If you skip question `i`, you get to make the decision on the next question.

- For example, given `questions = [[3, 2], [4, 3], [4, 4], [2, 5]]`:
  - If question `0` is solved, you will earn `3` points but you will be unable to solve questions `1` and `2`.
  - If instead, question `0` is skipped and question `1` is solved, you will earn `4` points but you will be unable to solve questions `2` and `3`.

Return the maximum points you can earn for the exam.

Example 1:

    Input: questions = [[3,2],[4,3],[4,4],[2,5]]
    Output: 5
    Explanation: The maximum points can be earned by solving questions 0 and 3.
    - Solve question 0: Earn 3 points, will be unable to solve the next 2 questions
    - Unable to solve questions 1 and 2
    - Solve question 3: Earn 2 points
    Total points earned: 3 + 2 = 5. There is no other way to earn 5 or more points.

Example 2:

    Input: questions = [[1,1],[2,2],[3,3],[4,4],[5,5]]
    Output: 7
    Explanation: The maximum points can be earned by solving questions 1 and 4.
    - Skip question 0
    - Solve question 1: Earn 2 points, will be unable to solve the next 2 questions
    - Unable to solve questions 2 and 3
    - Solve question 4: Earn 5 points
    Total points earned: 2 + 5 = 7. There is no other way to earn 7 or more points.

Constraints:

- `1 <= questions.length <= 105`
- `questions[i].length == 2`
- `1 <= pointsi, brainpoweri <= 105`