You are given two integers m and n representing a 0-indexed grid of size m $\times$ n. You are also given two 2D integer arrays, guards and walls, where guards[i] = [row_i, col_i] indicates the position of the $i^{\text{th}}$ guard and walls[j] = [row_j, col_j] indicates the position of the $j^{\text{th}}$ wall.

A guard can observe every cell along the four cardinal directions (north, south, east, and west) starting from their position, unless their line of sight is blocked by a wall or another guard. A cell is guarded if it can be seen by at least one guard.

Return the total number of unoccupied cells that are not guarded.

Note: A cell is occupied if it contains a guard or a wall. Occupied cells are not counted as guarded.

Constraints:

• $1 \leq$ m, n $\leq 10^5$

• $2 \leq$ m $\times$ n $\leq 10^5$

• $1 \leq$ guards.length, walls.length $\leq 5 \times 10^4$

• $2 \leq$ guards.length $+$ walls.length $\leq$ m $\times$ n

• guards[i].length $==$ walls[j].length $== 2$

• $0 \leq \text{row}_i, \text{row}_j <$ m

• $0 \leq \text{col}_i, \text{col}_j <$ n

• All the positions in guards and walls are unique.

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