The Sun-Wells of the Kessarine Waste

The Waterwardens of the Kessarine Waste are charting the great desert before the caravan season. Their survey splits the Waste into a rectangular grid of rows by cols squares. Rows are numbered 0 to rows - 1 and columns 0 to cols - 1. Some squares are sheer dune-cliffs that no traveller can enter. Some hold a sun-well. Every other square is open sand.

The Wardens will pay for one number: how far a stranded traveller could possibly be from water. A traveller moves one step at a time to a square that shares an edge with the current one (up, down, left or right, never diagonally). They may never step outside the grid or onto a cliff square. A well square is open and may be walked through.

Write farthestFromWater(rows, cols, walls, wells):

• rows and cols are integers, each from 1 to 100.
• walls is a list of [row, col] pairs, the cliff squares. It may be empty.
• wells is a list of [row, col] pairs, the well squares. It contains at least one entry.
• Every pair lies inside the grid. No square appears twice, either within one list or across both, so a well is never a cliff.

An open square is any square that is not a cliff. Well squares count as open squares. For each open square, find the fewest steps needed to reach any well. A well square itself needs 0 steps.

Return an integer:

• If some open square cannot reach any well at all, return -1.
• Otherwise, return the largest of these step counts across all open squares.

Cliff squares are never counted.