You are given `n` rectangles represented by a 0-indexed 2D integer array `rectangles`, where `rectangles[i] = [widthi, heighti]` denotes the width and height of the `ith` rectangle.

Two rectangles `i` and `j` (`i < j`) are considered interchangeable if they have the same width-to-height ratio. More formally, two rectangles are interchangeable if `widthi/heighti == widthj/heightj` (using decimal division, not integer division).

Return the number of pairs of interchangeable rectangles in `rectangles`.

Example 1:

    Input: rectangles = [[4,8],[3,6],[10,20],[15,30]]
    Output: 6
    Explanation: The following are the interchangeable pairs of rectangles by index (0-indexed):
    - Rectangle 0 with rectangle 1: 4/8 == 3/6.
    - Rectangle 0 with rectangle 2: 4/8 == 10/20.
    - Rectangle 0 with rectangle 3: 4/8 == 15/30.
    - Rectangle 1 with rectangle 2: 3/6 == 10/20.
    - Rectangle 1 with rectangle 3: 3/6 == 15/30.
    - Rectangle 2 with rectangle 3: 10/20 == 15/30.

Example 2:

    Input: rectangles = [[4,5],[7,8]]
    Output: 0
    Explanation: There are no interchangeable pairs of rectangles.

Constraints:

- `n == rectangles.length`
- `1 <= n <= 105`
- `rectangles[i].length == 2`
- `1 <= widthi, heighti <= 105`