You are given an `m x n` integer array `grid` where `grid[i][j]` could be:

- `1` representing the starting square. There is exactly one starting square.
- `2` representing the ending square. There is exactly one ending square.
- `0` representing empty squares we can walk over.
- `-1` representing obstacles that we cannot walk over.

Return the number of 4-directional walks from the starting square to the ending square, that walk over every non-obstacle square exactly once.

Example 1:

    Input: grid = [[1,0,0,0],[0,0,0,0],[0,0,2,-1]]
    Output: 2
    Explanation: We have the following two paths:
    1. (0,0),(0,1),(0,2),(0,3),(1,3),(1,2),(1,1),(1,0),(2,0),(2,1),(2,2)
    2. (0,0),(1,0),(2,0),(2,1),(1,1),(0,1),(0,2),(0,3),(1,3),(1,2),(2,2)

Example 2:

    Input: grid = [[1,0,0,0],[0,0,0,0],[0,0,0,2]]
    Output: 4
    Explanation: We have the following four paths:
    1. (0,0),(0,1),(0,2),(0,3),(1,3),(1,2),(1,1),(1,0),(2,0),(2,1),(2,2),(2,3)
    2. (0,0),(0,1),(1,1),(1,0),(2,0),(2,1),(2,2),(1,2),(0,2),(0,3),(1,3),(2,3)
    3. (0,0),(1,0),(2,0),(2,1),(2,2),(1,2),(1,1),(0,1),(0,2),(0,3),(1,3),(2,3)
    4. (0,0),(1,0),(2,0),(2,1),(1,1),(0,1),(0,2),(0,3),(1,3),(1,2),(2,2),(2,3)

Example 3:

    Input: grid = [[0,1],[2,0]]
    Output: 0
    Explanation: There is no path that walks over every empty square exactly once.
    Note that the starting and ending square can be anywhere in the grid.

Constraints:

- `m == grid.length`
- `n == grid[i].length`
- `1 <= m, n <= 20`
- `1 <= m * n <= 20`
- `-1 <= grid[i][j] <= 2`
- There is exactly one starting cell and one ending cell.