Collect all keys in grid

In a grid of characters, '.' is an open cell, '#' is a wall, '@' is your start, a lowercase letter is a key, and the matching uppercase letter is a locked door you can only pass once you hold that key. You move up/down/left/right one cell per step. Starting at '@', return the fewest steps to collect every key on the board (you may pass through doors only after picking up their key). Return -1 if it is impossible to collect all keys. Keys use distinct letters from 'a' to 'f' (at most 6).