Fruits are available at some positions on an infinite x-axis. You are given a 2D integer array `fruits` where `fruits[i] = [positioni, amounti]` depicts `amounti` fruits at the position `positioni`. `fruits` is already sorted by `positioni` in ascending order, and each `positioni` is unique.

You are also given an integer `startPos` and an integer `k`. Initially, you are at the position `startPos`. From any position, you can either walk to the left or right. It takes one step to move one unit on the x-axis, and you can walk at most `k` steps in total. For every position you reach, you harvest all the fruits at that position, and the fruits will disappear from that position.

Return the maximum total number of fruits you can harvest.

Example 1:

    Input: fruits = [[2,8],[6,3],[8,6]], startPos = 5, k = 4
    Output: 9
    Explanation:
    The optimal way is to:
    - Move right to position 6 and harvest 3 fruits
    - Move right to position 8 and harvest 6 fruits
    You moved 3 steps and harvested 3 + 6 = 9 fruits in total.

Example 2:

    Input: fruits = [[0,9],[4,1],[5,7],[6,2],[7,4],[10,9]], startPos = 5, k = 4
    Output: 14
    Explanation:
    You can move at most k = 4 steps, so you cannot reach position 0 nor 10.
    The optimal way is to:
    - Harvest the 7 fruits at the starting position 5
    - Move left to position 4 and harvest 1 fruit
    - Move right to position 6 and harvest 2 fruits
    - Move right to position 7 and harvest 4 fruits
    You moved 1 + 3 = 4 steps and harvested 7 + 1 + 2 + 4 = 14 fruits in total.

Example 3:

    Input: fruits = [[0,3],[6,4],[8,5]], startPos = 3, k = 2
    Output: 0
    Explanation:
    You can move at most k = 2 steps and cannot reach any position with fruits.

Constraints:

- `1 <= fruits.length <= 105`
- `fruits[i].length == 2`
- `0 <= startPos, positioni <= 2 * 105`
- `positioni-1 < positioni` for any `i > 0` (0-indexed)
- `1 <= amounti <= 104`
- `0 <= k <= 2 * 105`