There are some spherical balloons taped onto a flat wall that represents the XY-plane. The balloons are represented as a 2D integer array `points` where `points[i] = [xstart, xend]` denotes a balloon whose horizontal diameter stretches between `xstart` and `xend`. You do not know the exact y-coordinates of the balloons.

Arrows can be shot up directly vertically (in the positive y-direction) from different points along the x-axis. A balloon with `xstart` and `xend` is burst by an arrow shot at `x` if `xstart <= x <= xend`. There is no limit to the number of arrows that can be shot. A shot arrow keeps traveling up infinitely, bursting any balloons in its path.

Given the array `points`, return the minimum number of arrows that must be shot to burst all balloons.

Example 1:

    Input: points = [[10,16],[2,8],[1,6],[7,12]]
    Output: 2
    Explanation: The balloons can be burst by 2 arrows:
    - Shoot an arrow at x = 6, bursting the balloons [2,8] and [1,6].
    - Shoot an arrow at x = 11, bursting the balloons [10,16] and [7,12].

Example 2:

    Input: points = [[1,2],[3,4],[5,6],[7,8]]
    Output: 4
    Explanation: One arrow needs to be shot for each balloon for a total of 4 arrows.

Example 3:

    Input: points = [[1,2],[2,3],[3,4],[4,5]]
    Output: 2
    Explanation: The balloons can be burst by 2 arrows:
    - Shoot an arrow at x = 2, bursting the balloons [1,2] and [2,3].
    - Shoot an arrow at x = 4, bursting the balloons [3,4] and [4,5].

Constraints:

- `1 <= points.length <= 105`
- `points[i].length == 2`
- `-231 <= xstart < xend <= 231 - 1`