There are at most <code>4</code> cells with more than one stone.

Let <code>a</code> be the number of cells containing more than one stone, and <code>b</code> be the number of cells containing no stones. <code></code>. <code>b^a ≤ 6561</code>. Use this fact to come up with a bruteforce.

For all empty cells, bruteforce over all possible cells from which a stone can come. Note that a stone will always come from a cell containing at least 2 stones.