Minimum Sensors to Cover Grid

Description

You are given n × m grid and an integer k.

A sensor placed on cell (r, c) covers all cells whose Chebyshev distance from (r, c) is at most k.

The Chebyshev distance between two cells (r1, c1) and (r2, c2) is max(|r1 − r2|,|c1 − c2|).

Your task is to return the minimum number of sensors required to cover every cell of the grid.