A fly and $\mathrm{k}$ spiders move on a $2019 \times 2019$ grid. On their turn, the fly can move 1 square and the $k$ spiders can each move 1 square. What is the minimal $k$ for which the spiders are sure to catch the fly.
We show that 2 spiders suffice: when the fly moves to a coordinate, the spider on that coordinate replicates the movement, the other approaches.