The Creeping Stone of Halvane Gardens

The basilisk of Halvane Gardens is dead, but its curse was not buried with it. Some of the travellers caught among the hedges have already turned to stone, and the stone is spreading to the living who stand beside them. Before anyone goes in after them, the Halvane Wardens will pay for a reckoning: how long the curse needs to finish its work, or whether some of the trapped will be spared.

You are given grid, a list of strings of equal length describing the gardens from above. Each character is one square:

• 'S': a figure already turned to stone.
• 'L': a living figure.
• '.': empty ground.

Every hour, each living figure that is orthogonally adjacent (up, down, left or right, never diagonal) to a stone figure turns to stone. All changes within one hour happen at once, so a figure that turns during an hour does not spread the curse until the following hour. Empty ground never turns and does not carry the curse.

Write petrifyHours(grid), which returns an integer: the number of hours until no living figure remains.

• Return 0 if there are no living figures to begin with. This includes an empty grid.
• Return -1 if at least one living figure will never turn to stone.