Given an integer array `nums` and two integers `firstLen` and `secondLen`, return the maximum sum of elements in two non-overlapping subarrays with lengths `firstLen` and `secondLen`.

The array with length `firstLen` could occur before or after the array with length `secondLen`, but they have to be non-overlapping.

A subarray is a contiguous part of an array.

Example 1:

    Input: nums = [0,6,5,2,2,5,1,9,4], firstLen = 1, secondLen = 2
    Output: 20
    Explanation: One choice of subarrays is [9] with length 1, and [6,5] with length 2.

Example 2:

    Input: nums = [3,8,1,3,2,1,8,9,0], firstLen = 3, secondLen = 2
    Output: 29
    Explanation: One choice of subarrays is [3,8,1] with length 3, and [8,9] with length 2.

Example 3:

    Input: nums = [2,1,5,6,0,9,5,0,3,8], firstLen = 4, secondLen = 3
    Output: 31
    Explanation: One choice of subarrays is [5,6,0,9] with length 4, and [0,3,8] with length 3.

Constraints:

- `1 <= firstLen, secondLen <= 1000`
- `2 <= firstLen + secondLen <= 1000`
- `firstLen + secondLen <= nums.length <= 1000`
- `0 <= nums[i] <= 1000`