<p>You are given an <code>m x n</code> grid <code>grid</code> where:</p>
<ul>
<li><code>'.'</code> is an empty cell.</li>
<li><code>'#'</code> is a wall.</li>
<li><code>'@'</code> is the starting point.</li>
<li>Lowercase letters represent keys.</li>
<li>Uppercase letters represent locks.</li>
</ul>
<p>You start at the starting point and one move consists of walking one space in one of the four cardinal directions. You cannot walk outside the grid, or walk into a wall.</p>
<p>If you walk over a key, you can pick it up and you cannot walk over a lock unless you have its corresponding key.</p>
<p>For some <code><fontface="monospace">1 <= k <= 6</font></code>, there is exactly one lowercase and one uppercase letter of the first <code>k</code> letters of the English alphabet in the grid. This means that there is exactly one key for each lock, and one lock for each key; and also that the letters used to represent the keys and locks were chosen in the same order as the English alphabet.</p>
<p>Return <em>the lowest number of moves to acquire all keys</em>. If it is impossible, return <code>-1</code>.</p>
