Given an integer array `arr` and an integer `difference`, return the length of the longest subsequence in `arr` which is an arithmetic sequence such that the difference between adjacent elements in the subsequence equals `difference`.

A subsequence is a sequence that can be derived from `arr` by deleting some or no elements without changing the order of the remaining elements.

Example 1:

    Input: arr = [1,2,3,4], difference = 1
    Output: 4
    Explanation: The longest arithmetic subsequence is [1,2,3,4].

Example 2:

    Input: arr = [1,3,5,7], difference = 1
    Output: 1
    Explanation: The longest arithmetic subsequence is any single element.

Example 3:

    Input: arr = [1,5,7,8,5,3,4,2,1], difference = -2
    Output: 4
    Explanation: The longest arithmetic subsequence is [7,5,3,1].

Constraints:

- `1 <= arr.length <= 105`
- `-104 <= arr[i], difference <= 104`