Key Maze Solver

You are given a 2D grid of characters that represents a maze. Each cell in the maze can be one of the following:

• #: a wall that you cannot pass through.
• .: an open space that you can move into.
• @: your starting position.
• Lowercase letters (e.g., a, b, c, ...): keys that you can pick up.
• Uppercase letters (e.g., A, B, C, ...): doors that can only be passed through if you have collected the corresponding lowercase key.

You can move one step at a time in the four cardinal directions (up, down, left, right).

Your task is to implement a function that returns the minimum number of steps required to collect all of the keys in the maze. If it is not possible to collect all keys, your function should return -1.

Assume that:

• The maze contains at least one key and one starting position.
• There is exactly one starting position @.
• Keys and doors are paired (for example, key a opens door A).

Input: A list of strings, where each string represents a row of the maze.

Output: An integer representing the minimum number of steps to collect all keys, or -1 if it is not possible.

Example:

Input:
[
  "@.a.#",
  "###.#",
  "b.A.B"
]

Output: 8

Write clean, efficient code to solve this problem.