You are given an integer array `nums`. The unique elements of an array are the elements that appear exactly once in the array.

Return the sum of all the unique elements of `nums`.

Example 1:

    Input: nums = [1,2,3,2]
    Output: 4
    Explanation: The unique elements are [1,3], and the sum is 4.

Example 2:

    Input: nums = [1,1,1,1,1]
    Output: 0
    Explanation: There are no unique elements, and the sum is 0.

Example 3:

    Input: nums = [1,2,3,4,5]
    Output: 15
    Explanation: The unique elements are [1,2,3,4,5], and the sum is 15.

Constraints:

- `1 <= nums.length <= 100`
- `1 <= nums[i] <= 100`