You are given an array `nums` of non-negative integers. `nums` is considered special if there exists a number `x` such that there are exactly `x` numbers in `nums` that are greater than or equal to `x`.

Notice that `x` does not have to be an element in `nums`.

Return `x` if the array is special, otherwise, return `-1`. It can be proven that if `nums` is special, the value for `x` is unique.

Example 1:

    Input: nums = [3,5]
    Output: 2
    Explanation: There are 2 values (3 and 5) that are greater than or equal to 2.

Example 2:

    Input: nums = [0,0]
    Output: -1
    Explanation: No numbers fit the criteria for x.
    If x = 0, there should be 0 numbers >= x, but there are 2.
    If x = 1, there should be 1 number >= x, but there are 0.
    If x = 2, there should be 2 numbers >= x, but there are 0.
    x cannot be greater since there are only 2 numbers in nums.

Example 3:

    Input: nums = [0,4,3,0,4]
    Output: 3
    Explanation: There are 3 values that are greater than or equal to 3.

Constraints:

- `1 <= nums.length <= 100`
- `0 <= nums[i] <= 1000`