You are given an integer array `nums` consisting of `n` elements, and an integer `k`.

Find a contiguous subarray whose **length is equal to** `k` that has the maximum average value and return *this value*. Any answer with a calculation error less than `10-5` will be accepted.

**Example 1:**

```
Input: nums = [1,12,-5,-6,50,3], k = 4
Output: 12.75000
Explanation: Maximum average is (12 - 5 - 6 + 50) / 4 = 51 / 4 = 12.75

```

**Example 2:**

```
Input: nums = [5], k = 1
Output: 5.00000

```

**Constraints:**

- `n == nums.length`
- `1 <= k <= n <= 105`
- `104 <= nums[i] <= 104`