Given an array of digit strings `nums` and a digit string `target`, return the number of pairs of indices `(i, j)` (where `i != j`) such that the concatenation of `nums[i] + nums[j]` equals `target`.

Example 1:

    Input: nums = ["777","7","77","77"], target = "7777"
    Output: 4
    Explanation: Valid pairs are:
    - (0, 1): "777" + "7"
    - (1, 0): "7" + "777"
    - (2, 3): "77" + "77"
    - (3, 2): "77" + "77"

Example 2:

    Input: nums = ["123","4","12","34"], target = "1234"
    Output: 2
    Explanation: Valid pairs are:
    - (0, 1): "123" + "4"
    - (2, 3): "12" + "34"

Example 3:

    Input: nums = ["1","1","1"], target = "11"
    Output: 6
    Explanation: Valid pairs are:
    - (0, 1): "1" + "1"
    - (1, 0): "1" + "1"
    - (0, 2): "1" + "1"
    - (2, 0): "1" + "1"
    - (1, 2): "1" + "1"
    - (2, 1): "1" + "1"

Constraints:

- `2 <= nums.length <= 100`
- `1 <= nums[i].length <= 100`
- `2 <= target.length <= 100`
- `nums[i]` and `target` consist of digits.
- `nums[i]` and `target` do not have leading zeros.